Reinstalled the system and grr'd a lot.
* The IP conflict with someone's mac address of 00:55:de:ad:be:ef wasn't alone an issue (and on CMU campus, probably not part of the pwning, it just pointed me to it), but the fact that I couldn't run security updates, my ssh known_hosts values were all invalid, and other assorted messes... well... time for reinstall.
Edit later: Upon further consideration, it's thought that whomever did have that Mac Address had a pwned computer, and the appropriate people have been notified. No, it's not that the mac address got set up as "00-55-DEADBEEF", but rather that I *kept* having an IP conflict with them, following me through about 8 IP changes from DHCP. 8 different IPs, same mac address conflicting. Yah, there's a problem there.