Warning: If you are a dvorak user do not, I repeat, DO NOT upgrade OSX to Leopard 10.5.6. In fact if you use any keyboard layout other than US English you should probably wait as well. After upgrading you may find that you are unable to type using dvorak in some applications, or use some application shortcuts.
I got bitten pretty badly in ATOK, a third party Japanese IME, which was rendered completely useless. It wouldn’t recognize any input keyboard layout other than US English. Input layouts in Kotoeri did work fine however.
Now I’ve just finished reverting back to 10.5.5 from a backup. A quick web search turned up similar problems using Epclipse keyboard shortcuts, WOW commands and others.
Update 5/13/09: 10.5.7 is safe, apple has fixed this issue.