I recently solved the Word Completion problem I was having on my AT&T Fuze (HTC Diamond).
I received a new phone from AT&T in late December and found that Word Completion was not working, even when selected in Settings>Input. I found that the phone is configured out of the box with Word Completion 'off' which I found annoying. The AT&T reps on campus and in Redmond had no idea what was wrong and swapped out my unit for a new phone as they were unable to fix the setting. Same problem on that one, too.
But thanks to the wonder of Twitter and Live Search, I received an answer, and posting it here if any others are missing this ability, courtesy of http://www.htcwiki.com/thread/1720237/Word+Completion+Fix+for+6.1+ROM+Update?t=anon:
* After confirming that you have the box checked for "Suggest words when entering text" in the Word Completion tab found in Settings>Input.
* Open up a new email or text message.
* Go to the bottom of the screen and change your letter input method to Full QWERTY (using the down arrow next to the "pencil", "keyboard" , etc. icon).
* In the bottom left hand corner of this Full QWERTY keyboard, just to the right of CAPS/shift button, is a button to toggle between T9 and ABC.
* Click the button and make sure that T9 appears below ABC. Apparently the opposite appears as a default setting.
* Switch back to Block Recognizer, Keyboard or Letter Recognizer and Word Completion should be active for you!
https://twitter.com/HTC, please explain: why doesn't word completion or auto spell correction work out of the box on my AT&T HTC Diamond?