If you want to try out a FUSE-style Dropbox filesystem, I write one. It's available on Mac, Windows and soon Linux. [shameless plug]
The bit I don't quite understand about Project infinite is that you still have to manually decide what gets sync'd or not. It's also not a network volume, so things like virus scanners or search indexers can just page in-data? And if you want to offload data, you just move it from one folder to another which still eats space.