They've been selling an average of about 10m per year, so it'll be close. The economy is a wild card, as is a PS3 price drop.
If MS could cut the price again for next holiday, and release another Halo/GoW level app, it would likely cinch it.
However, 100m lifetime is far fetched. The 360 is hitting its 4th holiday, and should be peaking in sales (based on past consoles). Unless MS is planning on running the 360 for longer than I expect, they'll get one more holiday that truly belongs to this generation. By 2010, we'll be at least hearing about the NeXtbox, and in 2011, at latest, the console will be released. In between, we might see price cuts on the Wii, and maybe even a new Wii (or WiiHD) that could take a bit out of their sales. And as the PS3s price drops, you can expect that competition to be strong as well.
In the end, I'll be surprised if they can even hit 60m by the end of the console's life span.
