The OnStar equipment in your truck is basically a 3 watt analog cell phone with a built in data modem. (A typical hand-held cell phone is 500 mw, or 1/2 watt.)
When you are talking to OnStar, you are making a regular cell phone call though the cell phone provider, to the OnStar call center. Besides just talking to them, the data modem portion of the system can also transmit various vehicle information to the call center, and can receive commands from them.
When you use personal calling to call your wife, you are not talking to OnStar. You are simply making a regular cell phone call through the cell provider, to her. So, the people at OnStar cannot listen in, as they are not involved in the connection.
However, just like any cell phone, it is possible for someone with the right radio equipment to listen in on the conversation, just as it is possible to tap into hard-wired telephone lines.
So, is it absolutley secure? No, nothing is. But it's no different from any other cell phone. The key is that OnStar is not your cellular provider. It's just a call center, the number for which is essentially pre-programmed into your truck's system.