This is a hard one, if we go by best case senerio(HW and software sales take of $299.99 model? Best line up released WW?) I would put the ps3 as much as 32 million sellers by jan 1 09.
If we go by the rate its going right now, I suspect the ps3 should have about 27 million sellers by the end of the year.
2008 end of year predictions:
PS3: 22M
360: 25M
wii: 40M