Anyone who thinks Sony will be anything but last out of the gate isn't thinking clearly. The PS3 is a system built for a long term approach. Until it goes down to $200, until we see the true potential of the system and see how blu-ray penetrates the market, we will really not know the true sales potential of the system.
I think if Sony plays this right the PS3 could absolutely dominate hardcore gaming at around the time the next X-Box comes along. Microsoft could also sink their brand if they come out with a new system too soon, and try to push people to another expensive system they aren't ready for. 360 users might say to hell with them and go and get a $200 slimmed down PS3.
Wii 2: 2011
720: 2012
PS4: 2014