This does work now with recent firmware. I use it to make outbound calls from my Android cell phone over my wifi through the Obi using the Google Voice account on SP1. This way I can use my contacts and stuff through the default dialer, not the obion app, and not use cell minutes. I didn't try to make inbound calls through the Obi ring on my cell phone.
My setup:
Samsung Nexus S, firmware 2.3.6
Obi110 firmware 1.3.0 Build: 2586 (1.2.x should be enough)
Google Voice on SP1
On the android phone:
Settings > Call Settings > Accounts > Add Account
Username: myusername1234
Password: nonsense1234 (not used but required)
Server: 192.168.1.200 (use the ip of your obi, static dhcp is good)
Set as primary account: checked
Optional settings...
Port number: 5061
On the Obi:
Service Providers > ITSP Profile B > SIP
ProxyServer: 127.0.0.1 (not used but required, unless you have a real sip proxy here)
X_AccessList: 192.168.1.150 (optional security feature, put the static IP address of your cell phone here, or omit if you're not concerned)
Voice Services > SP2 Service
Enable: checked
X_ServProvProfile: B
X_RingProfile: B (probably not necessary, but haven't tested)
X_CodecProfile B (probably not necessary, but haven't tested)
X_InboundCallRoute: {myusername1234>(xx.|<:1555>[2-9]xxxxxx|1xxx[2-9]xxxxxxS0):sp1}
X_RegisterEnable: unchecked
AuthUserName: nonsense5678 (not used but required)
AuthPassword: nonsense9753 (not used but required)
On the X_InboundCallRoute the username must match the one on the phone. The "555" should be replaced with your area code. You could put a 911 route here if you want. i.e. {myusername1234>(911):li},{myusername1234>(xx.|<:1555>[2-9]xxxxxx|1xxx[2-9]xxxxxxS0):sp1}
I picked out the outbound rules from
http://www.obitalk.com/forum/index.php?topic=718.0 and
http://voxilla.com/2011/02/15/how-to-obify-your-ip-phone-no-asterisk-involved-2623 . The inbound stuff might work too, haven't tried.