You might want to check out FUSE for Linux. There are various programs built on top of it which allow remote filesystems to look exactly like local ones. Two that I use are curlftpfs and sshfs. It's really nice to be able to perform any of my computer's programs on these remote files, and it looks very similar to what Dropbox accomplishes. Of course, you would need to have an FTP or SSH login somewhere, but you can get free FTP access from e.g. Lycos, so that shouldn't be an issue.
In short, I guess I'm curious what separates Dropbox from using a free FTP service which is connected either through Windows' built-in Network Places or Linux's curlftpfs. There are obvious differences, but are they enough to warrant fees?
a linux port is doable (mac will come first) -- everything's written in python and was designed from the outset to be portable. although this isn't the initial focus of dropbox, a linux port would be interesting for maintaining small web sites or web apps -- instead of using scp/sftp or equivalent you could just modify the files on your desktop and have them synced to your web host.
+1 on being able to specify a folder inside the dropbox as a "server" folder, which means it has it's own ftp address, user, and password settings. Anything dragged there is automatically synched with that account. I thought of this as well as soon as I read that post about Linux support, as this would work with shared hosting without expecting hosts to install dropbox on their linux boxes. And the data would be backed up as well automatically in a third place (the drop box.) And, you'd have access to retrieve an older version of a file. This basically replaces the need for FTP clients if you also add a way to chmod the folders inside the "server" folder. Sam is 100% right.
All written in Python? I'd love to know a bit more about what you're doing if you can share. I put together a similar tool last year for myself (Windows-only) using NTFS' USN journal, but it sounds like you're doing something different.