There's little doubt in my mind that PS3 will hit 100 million. PS1 was at 70 million when PS2 launched, and it ultimately tapped out at 102 million.
But the PS1 was only 5 years when the PS2 launched and the in the past 12 months they had shipped 21 million of it. The PS3 is already 7 years old and has only shipped 10 million in the past year.
PS2 had way better legs, but it was too younger, 6 years old when the PS3 arrived, and the yearly shipment was also higher, 13 million (for a total of 107 million).
I think there is chance for both PS3 and Xbox 360 to hit 100M, but it's very small.