I don't think that anyone can compare the situation between last gen and this one.
I mean, who knows if the PS2 got to 5 mil within a year because of the release of major-selling game FFX (and price), and not the other way around? (that FFX sold a lot because of big userbase).
If that was the reason you can expect PS3 sells to spike drasticaly (I'm sure i've made a spelling mistake).
Look at Final Fantasy XIII's demo debut week, PS3 sells got from 15,500 the week earlier to 60,000, a spike of almost 400%!
I think that the full game can quadruple that precentage, since, well, demos that are bought usually sell less than the full game (look at GT4 demo and GT4 sells).
Now, the last thing that we need to know before we make proper predictions is the effect of the Slim, and who knows hot much it will widen the PS3 userbase in Japan?
Bet with Dr.A.Peter.Nintendo that Super Mario Galaxy 2 won't sell 15 million copies up to six months after it's release, the winner will get Avatar control for a week and signature control for a month.
