I just started using GTD on a BlackBerry 7290 recently. At first I was still using a Palm to complement my system, but I've now evolved 100% onto the Blackberry (complemented by and sync'd to MS Outlook) and it's the fastest/best GTD system I've had so far (since starting GTD 4yrs ago).

I just started posting some of the details of how I set it up (category structure, tagging,etc) on my "gtd/workflow management hobbyist" blog: http://stevebell.ws

hth,
-steve