I've been connecting to the service offered by fwdnet.net most of the time. I do run an Asterisk box on my LAN, but it is not publically-reachable. I suppose I could use that for testing at least. That would eliminate any possible NAT issue from the equation. I did set up a Gizmo account for testing that, and a WengoPhone account. Thanks for the suggestions.
[general] videosupport=yes
[n800] ... other settings ... nat=yes qualify=yes videosupport=yes canreinvite=no disallow=all allow=g729 allow=h263 allow=h263p