On current trends Wii might well reach 50%, but I doubt it will hold 50% and it will finish with less than 50%.
I doubt GTA IV or any individual game will make the difference.
The biggest threat to Wii is growth in HD TV and HD equipment and if BR sales start to grow (which will happen once format war is declared fully over). Looking at increasing growth of HD TV adoption its pretty obvious that many will be tempted to also have HD content to see on their expensive new TV. Now that might just be a standalone player, but it will likely boost PS3 sales and to a lesser extend 360 sales (I say this as really 360 doesn't hold up well right now as a choice for combined HD game console and player while PS3 does).
I was planning to get a Wii then a PS3 later but ended up getting new HD TV and reversed my order - now I'll get a Wii in a few months and it'll sit in one room with my widescreen CRT TV while the PS3 is already tucked below my HD TV.
Try to be reasonable... its easier than you think...
