Sorry but actually google stopped that feature or started to block it, as I first read about google voice sip address through the site known as OnSip and they had done some investigating into the google voice sip address and found out that google stopped it on march 8th or 9th. Here is the link for proof:
http://www.onsip.com/blog/leo/2011/03/09/google-to-launch-stand-alone-voip-service-reading-the-signs
I don't know if google completely stopped this as they are still trying to get the kinks out with sip and google voice as I don't work for them and can't speculate but hopefully they will allow people to start calling each other through sip by just typing someones gmail address from you google voice app on your smart phone and there computer or google voice on there smart phones start to ring. But hopefully that will be the next step, but just wishful thinking.