Sounds like a typical tarsnap set up. You can have a key that only appends backups, but can't delete them.
That is a good idea. I have considered having a network service like that, but I'm a little too paranoid. I want something that will survive heretofore unseen events, like a 0-day remote root exploit in the Linux kernel networking stack.
If such a remote root exploit exists, and the person that discovers it decides to write something like the Code Red worm from 2001, and also decides to have the worm erase all block devices, I'd currently lose every file I have and all of my backups, even though they are geographically dispersed. It's possible that even services like S3 and Dropbox would be wiped in such an event.
Once again, I realize I'm unusually paranoid in this regard, but I really don't want to lose my stuff.
Not everything is running on Linux you know, and therefore not everything is prone to the 0 day remote Linux kernel exploit. Get yourself other backup layers on other operatying systems, like Tarsnap, that runs FreeBSD, and you should get another level of security.
Don't Amazon services such as Glacier effectively guarantee you wont lose the data? (as in, they take proper offline backups and everything?)
This does rely on the FTP implementation being solid (vsftpd seems like a reasonable choice).