I think that they are waiting to see the performance of Dead Rising 1 on Wii before announcing the sequel. If it garners decent sales then they will go ahead with the sequel.
The sad thing is that there have been several quality games released by big companies that have flopped in regards to sales.
Okami, Zack and Wiki and Godfather are noteworthy ones.
I have a fear that these big releases for Wii like Tenchu, Deadly Creatures, HOTD and Dead Rising are all going to fail and if MadWorld and Conduit also falter then we can kiss a lot of possible games goodbye. I truly hope I'm wrong, but it could all crash quickly.