Let the hive mind of Engadget get that for you.
"I just switched to Sprint from Verizon about three months ago for the Pre. Then I went for the Hero about a week ago. Now, I miss my hardware keyboard and am thinking about switching to the Moment. I am still able to switch back to Verizon if I want and get the Droid when it arrives. Should I just trade up to the Moment when it comes out, see if I like it, and if not switch to the Droid? Or something else entirely? Help!"
I can confirm it works on my treo 650.
I have only rung myself very quickly, so I cannot vouch for it's quality or reliability, but I can say it definately makes the connection.
I am using sipgate.co.uk for the service. My treo is then connected to the internet over bluetooth to my macbook pro using "Missing Sync for Palm OS", which then goes over wifi to the telephone lines to my home phone.
It's hardly elegant, but it's getting there.
My mobile has an 0845 number! (for non brits, it's all proffesional like), and I can change it whenever. could be handy...