It seems that there isn't a perfect setup for GTD on the Mac. Lots of people like the desktop application
The Hit List, but it has no iPhone app.
Things has an iPhone app, but it lacks some cool things that The Hit List has, like hierarchical tasks.
OmniFocus is the most expensive application, and it has an iPhone app, but people say it has a steep learning curve and
isn't fun. And to top it off, if I did get an iPhone or iPod Touch for GTD, it has
security issues.
I'm going to continue using Entourage for email and calendar. For tasks, I'll switch from Entourage to The Hit List, because I do need a change. Instead of buying an iPod Touch, I'll continue syncing to my
old Windows Mobile PDA.