I have just read a couple of threads over on Android Forums, cos I knew I'd seen it mentioned before. The tip seems to be to use Handcent or Chomps SMS and this will allow you to rotate the screen to use a qwerty keyboard. The mention a few different keyboards from the market so I'll give you the links to have a look see. Also have a look in the box to the right of this screen More Like This as it will give you other threads on a similar theme.
http://androidforums.com/sony-ericsson-xperia-x10-mini/194329-best-keyboard-app-phone.html
http://androidforums.com/sony-ericsson-xperia-x10-mini/113936-qwerty-keyboard-sony-ericsson-xperia-x...
Hope you find what you are looking for.
