I don't have another keyboard, but I may have a new clue as to what's going on.

Doing some research online about the error message you cited, I found postings describing similar post-"sleep" problems with devices connected via USB. Like me, these people are having these issues only since installing Leopard.

Here are a couple of threads:
http://thelameleopard.wordpress.com/...-issues-again/
http://discussions.apple.com/thread....06301&tstart=0
http://discussions.apple.com/thread....sageID=6053232

And what seems to be a response from Apple acknowledging a problem with Leopard USB power management:
http://lists.apple.com/archives/usb/.../msg00055.html

Do you think this could explain the problem I'm having? I wonder if it would be worth waiting for the next update of Leopard before doing anything more, to see if the issue is addressed.