I can mostly confirm yegaoo's findings...

a voipbuster account can use the n800's default sip client (communications/internetcall) to make PSTN calls and video/calls to
other voipbusters..

works ok - when my n800 calls my wifes mac with xlite - have to enable the video with a click on the mac client...

I could not get gizmo to work with the default sip client at all - and found
an interesting artifact - that when I had all 3 services running with the
default client - that my battery went from full to 0 in about an hour...

The new n800 gizmo client could connect to the mac xlite - but video
was not supported.