[appliedluck]

I don't think I've seen anyone attempt to do it with the Dropbox API and your mobile contacts, but I'm pretty sure it could be done.

[ambirex]

I've been playing with creating a CardDAV server, which is pretty mush shuffling vCard files around. You could probably something similar with Dropbox.

[sayemm]

Yup, that's what I'm thinking... I think it would be cool to hack something up like that.