I noticed this with calls/SMS between myself and a local bike commuting pseudo-stranger whom I gave my GVoice info to, instead of my real phone number. He gave me his real number, but when I SMS'd him, it appeared to his phone via Google voice's SMS system. While it's cute and clever, it's not right. He was a bit confused, because he only gives his Google voice info to certain people. Knowing my background as a geek, he figured that I'd somehow compromised something just to show off, and was none too amused about it.
I think there's some confusion surrounding the SMS feature here. When someone sends a text to my GV number, it's forwarded to my phone from an arbitrarily assigned (406) area code number. This number is picked out of a pool of such numbers owned by Google, and it is permanently mapped to <i>that</i> person's number for <i>my GV account only</i>.<p>The reason this is done is so that I can then send texts to this newly assigned 406 number, and it will appear to have originated from my GV number. The GV system checks which phone the text is coming from, and says "okay, the phone sending this text is assigned to GV number X, and for GV number X, this 406 number is assigned to other person's phone number Y, so forward the text to Y and show it as coming from X."<p>It's complicated, but it ends up working out pretty well in my experience.
Meh, I actually prefer this. Sometimes I accidentally "leak" my real number because I unintentionally kill all my Android apps, disabling the 'use Google Voice for every call' feature. If the other person is using GV, my number won't be leaked in this case.<p>Seems like Google should make this a checkbox that's off by default, however.