I've fiddled with a lot of different systems over the years and, in the end, have settled with a fairly "standard" Outlook/Exchange Windows Mobile setup due to superior calendar sharing and mobile sync support. As I use Exchange, I can grab all of my data over GPRS or connect via WAP from other mobile devices. Exchange webmail is also solid and cross-browser compatible these days. All of my data lives on a server somewhere in New York so I can grab it on the road.
The David Allen company has a GTD Outlook add-in that looks promising:
Getting Things Done Outlook Add-In Support