This patch prevents malicious downgrades, which are still possible with DSA validation: suppose there's some (signed) version with a security hole. A malicious attacker could serve an appcast with that version's URL and DSA signature, but a higher version number, forcing the user to "upgrade" to the version with the security hole.
While I was at it, I fixed a bug that should have completely stopped .pkg installation from working since 1.5b1. Why didn't I hear anything about that? Does anyone actually use .pkgs? It still needs testing to be sure it works.