I'm borrowing a Verbatim external Bluetooth keyboard from a coworker, wanting to use it to type notes, using Colemak. I knew I could not change the software keyboard layout on my iPhone 4S to Colemak without jailbreaking, which I do not want to do.
But good news - changing the hardware keyboard layout on the iPhone 4S (iOS 5) is breathtakingly easy! I just went to Settings->General->Keyboard->International Keyboards and choose the first selection (in my case, labeled English). Then I set the Hardware Keyboard Layout to Colemak instead of U.S.
It only took about 30 seconds, and I was typing away on my iPhone with my external keyboard...in Colemak! Hooray!