I'm not sure if this link is already posted here, but let us all vote !

...Hi Steve Jobs. We now updated to the new iPhone Firmware 2.1 and it definitely made things go much smoother! But we all still miss one feature: Wed like to be able to disable the annoying autocorrection! At the time it is really not Apple-like. Please let your developers make a simple switch button to turn autocorrection off (they propably will do that in 20 seconds, right?). Thank you very much!...

http://please-let-us-disable-autocorrection-steve.com/