I have iPhone 4 on AT&T, iOS 5.0.1, jailbroken and running biteSMS 6.1 and SMS GV Extension 1.3.6
When sending SMS from biteSMS, it will first check if you are sending to a contact with iMessage. If so, it will send via iMessage, bypassing SMS GV entirely.
So your message will be free through iMessage, but it will come from your phone cell # or iMessage ID, not your GV #. It will also not show up in your GV history if you look at a browser.
Replies to you will come through iMessage, and will be picked up and displayed in biteSMS along with your other messages.
I don't see a way around this in the current version of biteSMS. It will always prefer iMessage when available.
Personally, I'd prefer that everything from me come from my GV #, not iMessage. So my workaround is to go to the Settings for Messaging, and turn OFF iMessage. Now biteSMS will skip iMessage and send all outgoing messages through SMS GV Extension, and everything will go to and from GV.
However, the downside to this is that if somebody sends me an incoming message to my iPhone #, it will no longer arrive as a free iMessage because I've turned iMessage off. Now it will arrive as a paid SMS to my cell #. I'll receive it just fine, and when I reply the reply will come from my GV #, but I have a lot of friends on iDevices who use my cell # to send me iMessages which I'd prefer to still get as free incoming messages, not as SMS.
In an ideal situation, biteSMS would add a setting letting me avoid sending messages through iMessage, preferring to use SMS GV instead for ALL outgoing, so I could leave my phone's iMessaging on to still receive free incoming iMessages.
Sorry for novel, hope that helps explain the behavior.