For a game like LBP, legs will be critical. We won't get a good idea of how it will sell lifetime until a month or so into 2009.
I'll be very surprised if it doesn't sell 2m worldwide *this* year - but if lifetime ends up under 3m, I think Sony will be very disappointed (considering its a "big" Xmas release).
Its also the sort of game that is hard to make a sequel for - as it (in theory) includes an indefinite amount of content (online). They could release level packs though, until the cows come home...
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