It seems one of theother niceties of Leopard (10.5.0, that is, haven’t upgraded to 10.5.1 yet) is that sometimes it will forget that the MacBook Pro has a keyboard, and you’re unable to type, even to the extent of the caps lock light no longer working. In my case the keyboard still appears as a USB device in System Profiler even when it’s not working, and there are no obvious log events happening in the Console Log. This never happened under Tiger. Others seem to be reporting this on Apple forums as well.
Update – Yes, an external USB keyboard works while the internal one is not responding.