If you reply to an SMS from your phone, the recipient will see your "real" cell phone number. However, if you reply to the 406 number that was assigned to that contact when they SMS'd you, the recipient will only see your GV number.
I'm going to try this experimentally. First, what is the actual point of 406, seeing to it that the call is deposited in my gv inbox? Otherwise, it strikes me that the return text is solely back to another google voice person's cellphone with no keying necessary from google voice. Is that right? Since you have to pay for text on cellphone anyway, what is the advantage of using google voice and the 406 number?