this is a very good idea. No other site I go to does this. Not that I will ever get to that point, but it's damn good gesture.
Here's a thought that may or may not be of interest. Another site I go to (truck site) has free membership just like here and they have a paid membership (14/yr.) paid gets you access to special forum features, more pm space, and a t-shirt. You also get your name colored (lol). This way you may cover some of your costs for the sponsorships and running the site. I also don't know how much t-shirts run by the quantitiy but maybe you could actually come out a little ahead.