It's not clear if you want to completely skip the pairing process or just avoid having to enter in a pin.
This SO Question has a number of useful answers related to both sides of that question, albeit with an emphasis on android. Short answer is, yes, you may be able to do what you're asking in limited circumstances.
The Wikipedia entry on Bluetooth has a number of interesting components, and I would recommend looking at section 7.3 regarding connections and bonding and section 8 regarding security.
Finally, this article goes over a number of prevalent hacks against bluetooth. They don't tell you how, and I wouldn't encourage exploiting those vulnerabilities, but it may be enough information for you to discover a legitimate way of doing what your describing. And honestly, it was pretty easy to google, so it's not like I'm giving away secrets here.