I'm not a trademark lawyer (and I'm certainly not offering anybody any legal advice hereby), but I am a practicing lawyer at a high-powered U.S. firm, and IP is one of those things that friends come to their lawyer friends for advice about these days. So a couple of facts might be useful if people want to discuss this further:
Trademark Infringement
As I said earlier, the key thing to know about trademark is that it hinges on consumer confusion. The whole point of trademark law is that somebody can look at a phrase or picture and immediately identify it with a particular company or person. A different company or person is not allowed to ride on the coattails of that recognition. I'm skipping over some of the details, but for present purposes, if a reasonable consumer would be confused by your use of a phrase or picture, you lose the trademark case. This is the key concept.
It doesn't matter that you don't intend to infringe on somebody's trademark. It doesn't matter that you aren't actually using their trademark at all (e.g., Sôny isn't a trademark, but it still infringes on Sony, which is). It doesn't matter that you aren't actually getting money, let alone profit, from what you're doing. It doesn't even matter if no consumers actually are confused. As long as a reasonable consumer would be, you can't do it (although of course, if you can prove that no consumers are confused, that's powerful evidence that no reasonable consumer ever will be confused).
Fair Use and Free Speech
I'll mention fair use and free speech since everybody always asks. First, fair use. Fair use does not mean non-profit use. It does not mean giving credit where credit is due. It does not mean personal use. It does not mean being a devoted fan with the best of intentions who even, dammit, arguably helps the holder of the trademark. If you ever thought fair use meant any of those, get those ideas right out of your head, take them out back, and shoot them.
In a trademark context, fair use essentially means accuracy. For instance, even though "Blood Bowl" is a trademark, you are allowed to put up an advertisement with the words "Blood Bowl" if you are selling bowls that are literally made of blood. You're just accurately describing your product. Similarly, it's not trademark infringement to discuss Blood Bowl online, such as saying, "I played a game of Blood Bowl last night ..." Again, you're just accurately describing what you did. Note that, even though fair use allows us to use the words "Blood Bowl" in discussion, it doesn't necessarily let us use it as a label. The discussions that go on in a forum are different from the URL of the forum itself.
Second, free speech. Free speech does not let you violate people's trademarks. The First Amendment (for us Americans) does not let you violate people's trademarks. It lets you discuss other people's trademarks. That is all.
Fan Sites in General
This starts to edge us into the realm of copyright law, as well as trademark - particularly something called derivative works. If you create something and write it down, record it, sculpt it, or otherwise fix it in some medium, you have (except for a few very weird exceptions which are not relevant here) copyrighted it. Copyright allows you to say who can and cannot copy what you have made. It also allows you to say who can and cannot create works based on what you have made. These are called derivative works (because they are derivative of the thing you originally made). To repeat, the copyright holder gets to decide who can and cannot make derivative works.
This is a big pitfall for fan sites, and indeed fan fiction and fan art. Drawing a totally original picture of a space marine is almost certainly a derivative work - GW gets to say who does that. Writing down the history of your ork Waaagh! is almost certainly a derivative work - GW gets to say who does that, too. Of course, there are original elements to both of those that you get the copyright in. For instance, the expression, the pose, the lighting, and the background of your space marine drawing are all yours - just not the space marine part of the drawing. You're free to draw something else with the exact same expression, pose, lighting, background, etc. Just not a space marine.
If you violate somebody's copyright, they are well within their rights to sue you to make you stop. They are also well within their rights to get any money you made as a result of your violation of their copyright. They are well within their rights to do both. They are well within their rights to do neither, which only means that they are doing neither for now, and will still be well within their rights to change their mind later.
A lot of fan sites are like the space marine drawing. You can keep the original stuff, just not the GW stuff - and if that means what you have left is full of holes and about as useful as a screen door on a submarine, too bad.
But again, what about fair use, and free speech, and fan appreciation, and ... and stuff?!
Fair use in copyright is a little broader than in trademark. It still does not mean non-profit, giving proper credit, a disclaimer saying, "No intent to challenge anybody's IP," personal use, good intentions, having the word "fan" in the name of the genre, or any of that, so if you still need to shoot a couple of those ideas, do that now. What fair use does allow in a copyright context is copying for purposes of critique, and parody.
So if you're drawing a space marine that makes fun of a space marine, that's okay. If you think about it, it kind of has to be - if you can't create a work based on another work for the purpose of making fun of it, you've essentially outlawed parody.
Similarly for critique. Imagine a drama critic reviewing a play, and saying in his review that the dialogue sucks. He is absolutely allowed to quote a few lines of dialogue in his review to make his point, even though that dialogue is copyrighted and the copyright owner hasn't given him permission to copy any part of the script. We are arguably allowed to quote GW rulebooks on the same principle when we debate the meaning of rules, even though those rules are copyrighted and GW hasn't given us permission to copy them.
Fighting Back
If anybody reading this is thinking about fighting back against cease-and-desist letters or similar action of this sort, there's two things you should know.
The first thing you should know is that you should talk to a lawyer who is expert in this stuff and who has a monetary incentive to advise you well.
The second thing you should know is that lawsuits, and the legal maneuvering leading up to lawsuits, suck. Do not get anywhere near a lawsuit without being willing, up front, to go through hell even if you win. This is not because lawyers are inherently mean people. It's not even because they're paid to be mean to you (although sometimes they are). It's because, even with the nicest, best intentioned lawyers, lawsuits inherently suck. I'm not saying that people shouldn't fight back in some cases - just that you should go into it with your eyes open.