Well going into the Holiday season I think Wii will be the obvious winner.
With that said, MS still has Halo 3 and while it may be lacking GTA IV for the holiday, you have to figure that Wii will be near impossible to get for the holidays. What's the alternative?
For players, PS3 really has absolutely nothing to offer for the holidays. Xbox will have Halo 3 and some other decent games coming out and with a smaller price point than the PS3 will probably default as the second system to get for the holidays (or even a back up system).
Not to say the lack of GTA will not hurt Xbox sales but I think out of all systems it's just another nail in the coffin for PS3.
Edit: To further this, PS3 will just have more issues with convincing developers to make games for their system as their share of the market keeps slipping away. As we go into 2008 Wii will pull away as the clear number one system and Sony will probably be left playing catch up for the rest of this gen.