Bug #2311 - please add a comment or vote for the bug as for me it happens far too frequently (at least 50/50 chance of no buttons following a device lock) when in 2007 I never had this problem. Oddly it doesn't seem to affect everyone.

BTW - the workaround to restore button functionality is to pop out the camera (unless the camera is already out, in which case you may be able to restore buttons by closing then opening the camera, or you could still be totally screwed).