Finally
Of course, you can achieve the same thing (more or less) with MobileMe, but for some unfathomable reason Apple insists on charging for it.
This is a good development. As a Dropbox user I can't wax more enthusiastically about it. I have Dropbox on my iPhone, and that's great 'n all, but to be able to sync all your info to as many machines as you'd like is just brilliant.