I know, I know, everybody's all atwitter over various iPhone software elements making their way to Windows Mobile. The latest is this keyboard over at XDA-Developers, which would be nice if you had a screen as large as the iPhone's. You don't, so this keyboard is going to be a single-index-finger affair. That's nice, it also utilizes Windows Mobile's predictive text, which has an advantage over the iPhone in that it will auto-complete a word for you with a tap. No, it's not as smart as the iPhone's "fix it after you type it" style, but it's still pretty nice.
However - I much prefer one of two other options: a physical keyboard or, barring that, SPB Full Screen Keyboard. It's $9.95 and works great. SPB did try to put in some of that fancy predictive magic that iPhone users have, but in my experience it's not all that useful. What is useful is that it's in landscape mode (it can do portrait too, but I'd advice against it). This, my friends, is eminently thumb-able. If you are one of the strange / lucky ones with a WM touchscreen device sans physical keyboard, this is what you want.
See, the nice thing about a Smartphone* is that if the built-in functionality isn't enough for you, you can hunt down and install 3rd-party applications to fill the gap. :p
This application won't analyze your next possible character like the iPhone does, but it is a skin for the Windows Mobile keyboard so you'll still get the native predictive word functions that you normally have.
Read: jkOnTheRun: Does this Windows Mobile iKeyboard look familiar?
( * Yes, I know this "the iPhone isn't a smartphone" thing is fast becoming a losing argument what with some decent web-apps coming out and the likelihood of serious updates from Apple in the near future. It's still mostly true (especially on a plane, hey?) and it's still completely fun to write. )