Right now it's looking almost certain, but the worst case scenario where the Wii starts to lose steam and only reached 70-90M isn't still entirely impossible.
I brainstormed something like this a while back:
20%: Third parties never jump over, people start caring about HD, next Nintendo console launches soon. Lifetime sales 60-100M.
60%: Most third parties jump over, non-gamers get moderately interested and keep buying games, similar lifespan to the PS2. Lifetime sales 120-160M.
20%: Third parties completely abandon the other consoles, attraction of non-gamers works better than anyone ever expected, lifespan is incredibly long due to virtually nobody caring about graphics, John Lucas elected President of the Universe. Lifetime sales 200M+.
The first scenario is looking less and less likely by the day, but that's all I dare say at this point.