I don't want to appear to be a big old policeman or anything but can I advise a bit of control when placing files on dropbox.
The main risks come from public folders. It's entirely possible for someone you've not invited to guess your public folder address, especially if they script something up. Also, if you're sharing folders with your mates, it's like sharing the same sexual partner, if they get a virus, so do you.
There are other issues too around the eternal life of shares, even if you delete or replace them on dropbox, particularly if you replace.
The risks are small (at the moment) but I wouldn't drop anything you care about or consider private on there.