I posted this in the main thread about MW2 sales numbers. I personnaly believe that Halo Reach will outsell MW2 on Xbox easily. Several things to factor. Halo 3 sold 3.8 million copies with an install base of 11.7 million (attach rate of 32.5%). MW2 sells (maybe) 5.5 million copies on an install base of ~33 million (attach rate of 16.67%). In the absolute worst case scenario, if Reach has the same attach ratio, the user base will be 7-9 million more, thus final numbers will be in the 6.5 million range.
It would be unrealistic to assume a similar attach rate as Halo 3, but if we were to split the difference at ~24%, that would mean 1st week sales would be in the 10 miliion range. This again may seem unrealistic (or is it?). At 20% attach rate, just 3 percent more than MW2, week one sales would be 8 million. At an attach rate of 10% Reach would still sell 4 million copies (and we all know that is not happening).
With regards to promoting the game, nothing so far has come close to Halo 3. From Game fuel from Mountain dew, to its own toy line, megablocks, to costumes. Heck 7-11 was promoting buying the game from them as well. Reach will be so massively promoted, with much more media coverage, that it is entirely possible that Reach could outsell MW2 in TOTAL sales if attch rate approaches 25%. My kids were 6 and 4 when Halo 3 was released, and they wanted Halo toys and wanted to be master chief for halloween, and they had never even seem me play the game. They were just massively aware of the media blitz. Now, 2 years latter, they are Halo junkies, love ODST's, love Sargent Johnson, love all the Spartans, have tons of Megablocks Halo toys. My older son loves Halo Wars. They are already asking when they can play Reach. All his friends, when they come over, all they play is some Halo game or another. Just like Star Wars, Halo is touching another generation, and its popularity is getting more widely noted.
The funny thing about Halo is that even though many people bitch and complain about the game, they still buy and play the game, complain some more, then play some more. Many of the die hard MW2 players were Halo2/3 players that migrated to the CoD frachise. But most of them will still slip the disc in and play it quietly and then come back to the boards and complain some more. 4- player co-op, more details about a great sci-fi story, imaculate match making, quite possible more inovations like Forge, etc, more tie ins with Halo Waypoint, will make the single player part shine. Likely the multiplayer will be more the same, but why break something that is not broken. Uptill MW2, Halo 3 is still the most played game on Live.
Reach will easily outsell MW2 on the Xbox, and has a realistic chance of outselling MW2 on all platforms.