You can actually make your public folder available through CloudFront by creating a distribution and pointing it at the Dropbox web servers. Unfortunately, it's a bit of a risky business, since all the other Dropbox users will be able to use your distribution as well.
Considering that DropBox is designed/developed for a end user backup service...
No, no, no. Dropbox is NOT a backup service.
It's just a user friendly backup service...
No, it's not. If you're connected to the internet and have sync turned on, and accidentally/mistakenly delete a file, that change is synced to your account. You can recover it for some period of time, but not indefinitely. Same thing with a virus/malware/etc. - changes are synced very quickly.
DropBox (and other file sharing sites) are blocked at my work, so I hope if this takes off, people use some sort of proxy.