I went ahead and picked up an unlocked Palm Treo 680. Perhaps eventually I'll transition to something else, but for now it made sense:
- My GTD setup is already set up in the Palm OS (no stress changing to a new OS)
- I am uncertain whether I'll be moving to the Mac OS from Windows (the Palm can be synched to either platform)
- Being unlocked, I can simply lift my SIM card out of my existing phone and move it over to the new one (I'm currently with AT&T and I like them).
We'll see what capabilities emerge over the next few years, but I'm glad I'm staying with Palm for now.