This isn't the easiest solution, but it works. Have GV set to forward texts to your mobile number and turn off notifications in GV. Then text everyone in your contacts list from GV. When they text you back, it will be sent to your mobile number, but the reply will be a 406 number. Save that number to the contact and then from now on, use your SMS app to text to the 406 number. This way you can use your default or Handcent app to text, but it will appear to the person you are texting that you sent it from your GV number. It's easier than it sounds and once you start doing it, it will make sense.