I had Wii at 18 million by the end of 2007 because I don't think they will ship more than 2 million in January-March 2008.
Regionally, I think the market will do this..
Americas - 40/52 weeks Wii>Xbox 360...but Halo 3, GTA, etc will push 360>Wii during October and November. There will be occasional other weeks when 360 outsells Wii as well.
Something crazy like 40 games could sell over 1 million copies in the USA in 2007 (although not all of them will)...Below are just the games that will release/are already out and will likely sell over a million. Others may too.
Zelda Wii, Wii Play, Super Mario Galaxy, Halo 3, GTA, Assassins Creed, MGS4, Mario & Sonic, Madden 360, DDR Wii, GH2 360, GH3 Wii, GH3 360, Mario Party 8, God of War 2, Lost Planet, GRAW, Diddy Kong Racing DS, Pokemon DS, Pokemon Wii, God of War 2, Madden PS2, Zelda GC, Resistance: Fall of Man, Wario Ware, Nintendogs, Gears of War, Wario Ware, Spider Man 3 (360), New Super Mario Brothers (DS), Big Brain Academy (Wii), Brain Training (DS), Lair, Killzone 2, Super Paper Mario, Bioshock, Alan Wake, Spider Man 3 (PS2), Crackdown, Mario Strikers
Japan - Wii will outsell PS3 & 360 every week this year. However, I think 360 may come close to outselling PS3 certain weeks. The DS will own Japan this year - best sales for 3rd parties, most hardware sold, most Nintendo software sold..Wii should have enough software released this year though to keep the market for at least another 18 months (Wii Sports, Wii Play, Wii Music, Wii Health, Galaxy, Brawl, Nights, Opoona, Treasure Z, RE: Umbrella, RE4, Mario Party 8, Super Paper Mario, Dragon Quest Swords, DDR Wii).
Europe - With Wii shipments primarily going to other regions, this will be the most hotly contested region in 2007. Microsoft's 50%+ marketshare will be gone in two months, and Sony probably has the best chance of winning this market of any region..But when Nintendo starts shipping more Wii's here, I believe this market could be split almost in thirds by year end...