You can, the PS3 takes a standard laptop HDD. The switch is fairly straightforward too, but obviously invalidates any warranty.
It supports externals too, but you can't install games to those.
Thing is, unless you have way too much money to buy or download games, 100Gb on a console will last ages. Less on the PS3 because of the installs, but even then not every game does it and most save files are only a couple of Mb. I don't expect my 80Gb to last forever (my 360's 20Gb is full) but it isn't going to fill up any time soon. But I'd happily upgrade to a big laptop hard drive when that happens.