Since anyone can compile the new GPU (cuda) miner, it is totally worthless dealing with CPUs.
nvidia miners are not many,,, and they dont have big farms. anyone that builds a big mining farm uses ATI.
You don't need farms. Lets assume cudaminer 10x faster than cpuminer (it should be even more faster).
Lets assume there are equal number of cpu and nvidia miners.
Than cpu users will have only about 9% of coins, and I think in reality they will get even less.