I wrote about T9 implementation on last year:
http://winmobiledeveloper.blogspot.com/2008/06/how-can-i-add-t9-to-textbox.html
And I found a little different way to enable T9 on your textbox.
There are more special feature, e.g. you can associate PIM Contacts with the TextBox.
Here is blog post from Christopher Fairbairn with sample code:
Making auto suggestions more appropriate for your textbox
