USB keyboard for me, don't know about the OP. I used a flash installer created based on what I think was 10.7.2, but I forget as I made my installer a while ago.
Are we talking USB or PS/2 keyboard and mouse? How did you install Lion originally - iAtkos, flash drive method, hard drive method?
Try this: IOUSBFamily.kext Rollback
Maybe this was something one of the post-boot tools I used took care of for me once I was booted up.. But I don't think I ran one as I could not login with a not so working keyboard. The odd thing was I did not have any problems with my USB mouse.