I just wanted to chip in with my two cents.
I currently own the mid-2010 model unibody mac mini as well, and wished to use a mic with it. Discovering that the line-input was unamplified, and therefore far too low to be used with a standard mic I did some hunting.
A USB interface is probably going to be the easiest solution. The iMic is a perfect example of the concept, but similar USB sound cards can be found quite inexpensively elsewhere on the web. However, I discovered - somewhat by accident - that the audio out port (despite having no documentation to indicate it anywhere that I can find) does in fact have a four-contact combo headphone-mic interface. I have successfully gotten a standard PC two plug headset to work with an adapter like this:
http://shop.steelseries.com/index.php/accessories/steelseries-mobile-device-adap ter.html
That should mean that any headset designed for use on any other combo jack (iPhone, Blackberry, etc.) should also work on the 2010 mac mini!