In the short term (next 6 months) I think it is fair to say that the Wii will continue to dominate the market and sell at (roughly) the same rate as the XBox 360 and PS3 combined; after GTA4 I don't think you can really argue that any game will motivate gamers to start buying either system at its current price to the extent that would be needed to change this.
In the mid term (next 6 to 18 months) I think that the best case for the PS3 and XBox 360 is to hold 45% to 49% of market (combined) and for one of the systems to have only been outsold by 30 Million units by the Wii.
After that it is possible to see the PS3 and XBox 360 attempt some sort of a comeback, but it would take many years to close the gap on the Wii; and if they were successful and started to catch up to the Wii, Nintendo could soon release a new inexpensive console which would erode the selling features of the PS3 and XBox 360.
