Project Infinite, announced at Dropbox’s Open London event wants to give you the best of both worlds. The idea is fairly simple: you can view all your Dropbox files right from your PC, but your computer will only download files as it needs them.
For a while there I was wondering if Dropbox would do anything to update their service. This would be a great benefit as I don't need to have my whole Dropbox synced to my MacBook all the time. Just what I need, when I need it.