You cannot disable journaling on the boot volume or on your Time Machine volume. That's why the button would be disabled for such volumes.
[Standard "I'm running OS X 10.5.8" caveat applies to this response.]
I can disable journaling on the boot volume by option-clicking the File menu, as Jon suggests above:
2/28/12 6:23:36 AM Disk Utility[2015] Disabling Journaling for volume “Esoteric_Diskâ€
Once journaling is disabled, the button becomes enabled as expected. (In Leopard, anyway, the button doesn't seem to toggle between the two actions, even for non-boot volumes, despite the fact that delving into Disk Utility's package contents reveals
/Applications/Utilities/Disk Utility.app/Contents/Frameworks/DUSupport.framework/Versions/A/Resources/DisableJournal.tiff as well as
EnableJournal.tiff. )