Great news. Eventually we will have a fully compatible softphone!
From bestkungfu weblog:
I've been waiting with bated breath for a voice-over-IP phone on my PowerBook. I have a Vonage account with a Cisco terminal adapter, which works great when I'm at home, but is a little bulky to carry around with me. So I signed up for a "softphone" with Vonage, a second account that works with a connected PC. Unfortunately, for the first couple of months, I could only get it to work on Windows.