This doesn't seem to be documented anywhere online that I can find, so:
Allan informed me on IRC that the mate command doesn't work from inside screen in Leopard.
All is not lost, however. I discovered that if you install screen via MacPorts (I had to 'port sync' first, then 'port install screen'), you can use the mate command from inside screen with no problems. Hopefully this will save someone else a few minutes of fruitless searching.
- Andre