TimeMatters + BillingMatters
Take a look at TimeMatters and its time-keeping/billing counterpart, BillingMatters.
www.timematters.com
They've just released version 6, which is a major upgrade from previous versions. Rock-solid program with a feature list too long to post.
You'll find a very active user forum, primarily consisting of attorneys and accountants. I've used TimeMatters for years to manage my case-load (attorney) and don't know what I would do without it. Combines contact, ToDo, case ("projects), events, notes, email, phone calls, into one database that lets you see everything you've ever done for a client/project. Tightly integrates with MS Office and Acrobat/PDF.
There are a few TimeMatters users on this forum that need to be drawn out of the wood-work to tell how they've used GTD with TimeMatters, so maybe this will do it?!
I have no affiliation with the company other than being a satisfied user (my firm has a 10 user license). Oh, it also syncs with your Palm/PocketPC and you can set up a remote laptop version that does synchronized bi-directional data transfers.