Engadget slips the news that Vonage ships its F-1000 Wi-Fi handset: The handset will work over hotspot networks allowing Vonage customers to use their service while roaming. Boingo and Vonage had a deal in place to test out VoIP over Wi-Fi hotspots, but it's unclear here in the late evening how that ties together. Also, Vonage's site doesn't yet list the announcement, so we don't know if they've partnered with various networks to ease authentication.
Authentication, or providing credentials that let you use a given hotspot network--whether a paid login or a WPA encryption key or handling 802.1X, even--is the biggest stopping block in allowing VoIP over hotspot to work. A technology like EAP-SIM, which would use a GSM SIM module to authenticate, might be one method of logging in. But it requires every hotspot or hotspot network that wants to allow this sort of connection to build the back-end to handle it.
There's slightly more coverage at USA Today which provides more background and notes the phone will probably be sold for about $100, but doesn't answer other questions. The article does make the point almost indirectly that the Wi-Fi phone could be a replacement for a phone-line adapter that Vonage now offers.