I upgraded the MacBook Pro to OS X 10.5 last night and today when I was trying it out I noticed to my horror that the excellent Mail Act-On extension didn't seem to be updated to work with Mail 3.0 (the new version in Leopard). Eeek!
I use Mail Act-On to have some quick key-strokes to archive mail. For example I have one rule that'll put mail in archived "Friends" and "Family" folders for personal mail. Another to put mail in various "Clients" folders. One for generic "Saved perl.org mail", etc etc. Without it I know my inbox would overflow even more than it does already.
The developer wrote today though that it will work. Yay. I also found on a beta tester mailing list that there's a 2.0 version in the works. So: Mail Act-On isn't dead and should work in Leopard. Phew. I was just about to post-pone upgrading the PowerMac... :-)