I would think it would have a shot...
What will be the big selling PS3 exclusives in Americas? Just God of War 3 and Gran Turismo 5. I think both will sell in the 1-1.5 million range. Other than that you have Heavy Rain, 3D dot Heroes, Modnation Racers, MAG, and Socom Move. I think those all sell maybe 300k.
If you add it all up Reach probably needs to sell 5-6 million to beat every PS3 exclusive. I think that's possible. Halo 3 did that. Reach should do more. It will have 4 months to sell also.
Am I missing something or way off base?
I'm assuming Resistance 3 will not be announced for this fall, and that The Last Guardian wont make it either. Especially the latter prediction seems very safe.
BTW I'm not really counting Move games in this though, since none have been announced and it's too much of a wildcard. Just hardcore titles. Socom counts though.