June 16, 2013: This seems to have changed since 2011. I couldn't find any current documentation.
Did some work with a packet sniffer today (Wireshark) dialing out from the Google Talk Plugin under Firefox 21.0 running on Windows 7.
Looks like the call setup is now done through standard HTTPS (TCP 443).
In several test calls, RTP (voice) packets were sent to UDP destination port 19305. I have no idea whether that will stay the same.
The Differentiated Services Field (DSCP) of the RTP packets is 0, so that can't be used for OoS setup.
On my router running Tomato, in the OoS settings, I have classified application Layer 7 (RTP) as Highest. Unfortunately it is not detecting the Google Voice packets; they were passing as Low priority. So I added a new entry for UDP Destination 19305 and the RTP traffic was properly prioritized.