I remember that the WFB used to be part of the 40k universe, just separated by the aforementioned Warp Storms (possibly in the Maelstrom, or even the Eye), which is why Sigmar was a rumored Primarch.

I think at some point, GW announced that the WFB and 40k universes were totally separate, though. I can't remember where or when the announcement was made, though. Possibly in a White Dwarf? Anyone else know what I'm talking about?