A while (a long while) back I was readingxkcd's blog post, and while some of the comments were interesting, I had my own take to it. My idea is to just use 9 keys, say, WERSDFXCV, as the keypad, and type as you would on a cellphone.
There are several advantages to this over the mirrored keyboard. One is that there is no new mapping to learn; everyone (well, everyone with a cell phone) would know the configuration. The second advantage is that there would be less finger movements for each word, which (like the last point) amounts to the system being easier to learn. In the mirrored keyboard, each key still only has one letter, so your fingers have to move over the keyboard more. With the text messaging keyboard, each key represents 3 or 4 letters, so only a few fingers have to move, and only one row at a time.
I forgot to mention that this system has to be used with a word list and a regex search, as mentioned in the xkcd post. On normal phones you have to press a key several times, which adds up to a lot more key presses after a while. It would be much better if you press each key only once, having each "number" stored in a buffer, and on word separating keys (spaces, commas, periods, etc.) the system would flush the buffer by matching each key with the possible letters, and extract out the word you meant.
For those who are not comfortable at all with their left hands, this can even be changed to a right hand dominant configuration. Instead of turning certain keys into the num pad, you use the mouse instead. Your left hand would simply be holding down a special combination of keys - say, control-shift-alt - and you would move your mouse in the direction of that number. 1 would be upper left, 2 straight up, 3 upper right, and so on. 5 would be clicking the left mouse button. This is similar to mouse gestures for Opera and Firefox, and could also use the same configuration button of the right click as the special key, so the left hand doesn't need to be on the keyboard at all.
The are two disadvantages I see in this system, ignoring the mouse gesture variation for now. One: there is no way to get punctuation in. This is a pain on cell phones, and I don't see a quick way for doing that. Not even a regex search would help, since the computer will need to know the context and meaning of what you're saying to determine which punctuation to use. The second flaw is that the regex search for words is inaccurate, and for small words will often require a choice. For example, both "bake" and "cake" use the keys 2-2-5-3. Again, this cannot be solved without the software knowing more about what you're writing about.
Personally, I just move my right hand off my mouse and type normally.