I have a bash script that does this. I set it up as a queen beecon widget that runs every time my n900 connects to a network. When it sees that it is connected to a specific wifi network it enables my sip account, changes my forwarding options in google voice, and disables the cell phone part of my phone (to save battery).

The only issue is that it uses my data connection to change my forwarding options after I leave my house, but as long as you click on the queen beecon before you leave, it would work.

Let me know if this doesn't make sense, or if you want me to post the script.