Batteries leaked in keyboard.
My keyboard batties leaked and now I am not physically able to open the battery cover. iMac can not be used without keyboard.
iMac (27-inch Late 2009), Mac OS X (10.7.3)
I had the same problem - eventually solved with a hacksaw !
Making a small cut in keyboard body beside cover and on same axis as the batteries are inserted - this 'opened' or 'relaxed' the external thread allowing the cap to be unscrewed.
It turned out the battery had leaked and the cap still needed a bit of manipulation before it came free, but now batteries changed and typing on my wireless keyboard again.