Many months ago I joined the Twitter nation (tthrush, but don't expect much, I mostly lurk) and eventually added a Twitter app to my cell.
At first I tried Twobile, but the interface was a bit lacking and the it could've been little quicker.
I've since settled on Pocketwit, another free client, this time from Google. The interface is elegant and the usability is top notch. If you're using a Windows Mobile phone, I highly recommend trying this out.