For those who consider to implement GTD on blackberry, I suggest you take a look at the GTD outlook plugin here:

http://gtdsupport.netcentrics.com/buy/

Try their 30-day trial version. I think it's a huge help for any one who values their time and efficiency with emailing. The TODO Matrix is nice, but too bad they don't integrate with outlook. After comparing both, I found it's much easier to use GTD Outlook plugin than trying hard on TODO Matrix. Both will do the job, but the Outlook Plugin is much more efficient. Try it yourself and let us know what you think.