I think how we define legs is entirely suspect on the game.
If Halo3 does move around 3 million copies the first week, how can it have long legs? I'm definately not in the camp of "everyone that wants Halo3 will buy it first week, and/or have a X360 already", but here are the facts:
Selling 3m units first week is beyond insane. If it sold 3m copies f/w, that'd be the #2 game of the year in 1 week. So 3m copies for ANY game within a 1 week timespan is ungodly.
Halo3 will not have a good multiplier due to this. At 3m units in a week, it's near impossible for ANY game to have a great multiplier.
However, having said this, what I'm thinking is this:
Halo3 will sell around 2,500,000-3,000,000 units in 7 days or less.
The week afterwards, sales will drop to around 450,000-500,000 units. This is an insane drop (around 80%), then start slowing it's decent down. Is this bad? Yes, if your talking a typical game. But Halo isn't a typical game. Selling 500,000 units during a non-holiday second week would be ungodly (at 500k, right now, H3s second week would be bigger than almost every games first week this year).
After this, it should see sales in the 100k to 500k range the rest of the year (think Gears in December numbers), then trail off slowly. Is this bad? Heck no. But we must really define what are good legs: Is it just a good multiplier (ala Wii Play), or strong sales regardless of a multiplier. IMO, I consider a game like Halo2 to keep selling 6k units a week, 143 weeks after the game has been out (and system discontinued) of far greater importance than the fact it has a 3x multiplier. Games like that show that they are true blockbusters.
Gears of War is proof at great great sales and multipliers: 9 months after release, and gangbuster sales during Christmas (1m sales in Dec in 5 weeks), that it's still doing insanely well: It's sales have gone up, not down, week over week since the price drop and such.
Put me in the "Halo3 will sell 7m+ copies in the US/NA" club. If not more. I think Halo3 might contend with every title this generation (next to SMG) for top-selling exclusive game for a console in the US.
Back from the dead, I'm afraid.
