I couldnt find a "decent" warhammer forum.
There is one that a lot of the tournament scene use. That girl i played at the GT said she uses it alot actually.
warhamemr.org
I looked at setting up a sister site to run alongside heresy at one point. It'd be pretty cool having 2 sites slaved to the same database.
The fantasy forums on here arent likely to pick up much with such an obvious 40k theme and title.