defaults write com.apple.finder AllowCutForItems 1
and then restart Finder. When files or folders are selected in Finder, the Cut command will be active in the Edit menu, and present in the context menu.
Unfortunately, all it does today is a move-to-trash, and there's no ability to paste and have the file retrieved and put in the active Finder window or selected folder. So I don't know if this is a feature half-done, a feature dead-ended, or just a redundant command disabled because it would confuse people.
I kind of hope it's half-done and will be available in 10.5. The move-to-trash as an interim step appeals to me, because while the superficial user experience is consistent with any other cut operation, it's less inherently destructive.
[robg adds: I tested it and it works (half works?) as described. If you want to disable it again, just repeat the above command but replace the 1 with a 0. This will leave the value in your Finder's preferences, but it will be disabled. If that bothers you for some reason, you can delete it entirely by typing defaults delete com.apple.finder AllowCutForItems. If someone can test this on 10.3, please comment -- I don't have a bootable 10.3 partition at the moment.]

