I didn't even notice the ads until I saw this post, thanks Vangogh! ... (just kidding, of course)
I think you've done this in good taste. The ads seem to be relative to the forum and not distracting at all. I too appreciate the fact that you are communicating with the members.
We're probably all glad they are not bright flashing banners and huge drop-downs that cover the text you're reading or those little popup links throughout the text, you know, the ones that keep opening if you happen to hover nearby.
Yup, it costs a lot to run a nice site. Seems like a very reasonable balance to recoup some if not all of that cost. I ditto the kudos!