I think if you check Daylite (www.marketcircle.com), you'll find everything you need for a Mac application that can be readily adopted using GTD principles.

Feel free to drop me a line at weblistlrk@mac.com if you'd like more direct insight into that program, how to set it up, etc. Their web site, however, has demos, screen shots and the like and should give you a pretty good idea about the program.

Great for GTD!