It would be sweet if you offered weekly paid "trial" gallery spots. You could make them available only when a spot has remained unsold for x amount of days (in order to not cut in on the bigger orders as much).
I know, I know, the administrative costs would increase with less guaranteed payout, the setup time on everyone's part would increase, you'd sometimes have to postpone bigger orders while waiting for smaller ones to go through, etc...
But there's definitely a lot of webmasters like me out there who make a living at this, yet don't have a huge advertising budget (about $2k per month max), so it's hard for us to dump the whole lot into spots on a site that we've never been listed on before.
Anyway in summary, I know why you don't do this and I probably wouldn't either if I were in your shoes, but I would love it
That is all.
-Phil
PS - Mark you still owe me a beer for coming up with your slogan ;)