The ones that are available (and soon to come)... NextAction, "Falcon," nAr, and others... that try to sell themselves as GTD programs are exceptionally limited (and a waste of money) as compared to what you would find on other platforms. These programs seem to be just trying to exploit the concept for a quick buck rather than providing anything meaningful. Conspicuously, they don't seem to offer trial versions, either.

TodoMatrix (currently), and potentially PocketInformant (by the end of the year) are really likely the best options for robust and meaningful GTD implementation on the Blackberry.

Another alternative, especially if you are running Windows and Outlook, is MyLife Organized, which has a nice deskside component, syncs well with Outlook (and thus, the Blackberry) and is viewed by many to be a great way to implement GTD.

(As an aside... it's really odd how we're suddenly seeing all of these GTD questions... I'm beginning to wonder if they're more advertisements than real queries...)