I suppose if you're familiar with a certain layout, and you've untrained your brain (or perhaps never learned) QWERTY, it could be valuable. But I can't imagine switching layouts for becoming faster at txtn ur bffs will do you any good.
There's a third party app for Windows Mobile since 2004 that lets you swap keyboard layouts. Although the OP might be considered "new news" again since the inception of faster, more interesting smartphones like iPhone, Android, Dream, etc.
*queue Windows & Smart in adjacent sentences jokes here*