PS3 reaching 100 million might be stretching it, but it's a possibility. Considering how Sony has already reduced production costs in half and around August will reduce it even further in just over a year of its life. Still, in reference to the comment by dsoverpsp, the reason why it makes more sense for the PS3 to achieve 100 million and not 360 is because the PS3 is selling faster and better in the amount of time that it's been out than the 360, and that's with the hefty price tag. Sony is known to keep a long life to their systems, just look at the PS2 and the PS1 before that. The price will become reasonable and once quality titles flood the market, people will be more interested in it.
While Microsoft has done leaps and bounds better than last gen's attempt, they've still much to prove and sales are slipping, that's why 50 million might be stretching it.