I always found the iPhone keyboard to be the best touch screen QWERTY keyboard around with light touch and great predictive technology. I think Microsoft may have raised the bar with the new Windows Phone 7 keyboard that has that same light touch with great predictive text, auto correction, and easy tap to correct technology. Check out my video walk through using the keyboard on an HTC Surround device coming to AT&T soon.
As you can see the ability to tap an incorrect word and have the software give you likely alternatives is quite powerful. I love the way the words are auto corrected and shown in bold letters while giving you several choices to quickly tap and choose and save you from entering all the letters in a word. You will find that your library grows dynamically too so a unique word you spell a few times then appears as an accepted word on your own device.