You can debate that forever because everyone defines it differently.
But, bottom line is KOTOR was GOTY 2004 just about everywhere, it sold 2million on Xbox which puts it right up there with Fable, PGR, and Splinter Cell.
It is certainly capable of sellin systems and being a HUGE hit. If KOTOR could sell 2million last gen, Mass Effect should be able to destroy that number with the current purchasing trends on 360. You could argue it was the SW IP that sold the game, but the number of flopped SW games sortof goes against that argument.
And assuming it does sell better than KOTOR did, how can you saya game that sells to 20-30% of the install base is not a system seller?
p.s. If you want to go anecdotal, I would've bought a PS3 by now if Bioware were making their games on that system. KOTOR was the best RPG I've played in my life (Yes it beats FF6, FFX and Chrono Trigger), and the mere fact that Bioware was making exclsuive 360 games sold me the console day 1.