Hi
I have the Exact Same Problem,
On my MBP with Leopard.
There are 3 quick fixes(it doesn't resolve the issues permanently), but Apple needs to fix it :
1) Try Plugin in USB device and make sure it initialises like a USB Stick. (It works for me)
2) Press "Numlock" repeatedly for about 3-4 sec. then the obviously the Light comes on and the Keyboard is work.
3) I have a Bluetooth Mouse if, I switch it on and off ... as soon as it picks up the mouse the Keyboard works again.
Lastly tell Apple about it here:
http://www.apple.com/feedback/macosx.html
Hope it works.
Karl