In case you missed it, there are a huge number of problems with the upgrade to 10.4.3. See the Apple discussion forums. Unfortunately, I didn’t read them first, so since my upgrade I can’t run Mail, NetNewsWire or even Software Update.
Update: Things are finally back to normal and 10.4.3 is working. Thanks to all who emailed with suggestions. As best I can figure out, the problem was caused by a failure during the installation process. It left things in limbo. I was able to repair the permissions and re-install the update from a standalone disc image rather than through Software Update.
Still, there are an unusual number of people reporting problems with this update, so just make sure you backup everything first, as always.