Thanks to the hard work of Tim Abbott and the FTP masters, Sage can now be found in the unstable repository. Tim deserves a whole bunch of kudos for his determination in getting Sage into Debian—it involved a lot of working with upstream to resolve a variety of problems that made life difficult for packagers, and, as far as I can tell, a non-trivial amount of time. My involvement in the job was fairly minimal—just reviewing and uploading. There are still a few issues with the packaging that are being worked out with upstream for the next version before sagemath can migrate into testing (after Lenny, of course), but the package should be at least functional.
If you were looking to give Sage a try before but gave up because it wasn't in Debian, now's your chance to check it out! (The same goes if you're interested in free mathematics software but hadn't heard of Sage before.)