Thinksecret reports that Apple has already seeded Mac OS X 10.3.5 to developers today.
According to the rumor site, the new seed has changes in Mail, Safari, OpenGL, Networking, Graphics, Bluetooth, Firewire, and Audio/Sound.
The newest revision of Panther is said to be running on the (as yet) unreleased dual 2.5GHz PowerMac G5s.