Wow... Something must be off (in your testing)... Maybe you are forwarding SMS to your phone # from google voice? I would love for this to be true and would totally be all over it if we can get that kind of functionality.

Currently I get google voice SMS via instant email delivery on my gmail account. You can reply to that email and it will send the SMS back properly to the sender. And now with threaded Email in PR1.2 it sort of does the job, but direct integration in Messaging would be best