In March I mentioned that I was looking into this, and I wanted to let people know that we now have a website:
and the ability to buy or sell ads on ArtCal plus 9 blogs so far. For ArtCal, it's only for the banner ad at the top, or a 125×125 button under the left nav. The ad network software isn't really set up to manage something like the gallery ads on the right at the moment.
I'm sure some of my readers are interested in buying an ad, or signing up as a blogger/publisher. There is a signup link if you wish to serve ads on your site. You also will have the advantage of seeing some of your posts show up on the home page of the newly redesigned ArtCal which launches this month.