I have seen this on a few websites, though sadly I cannot recall which blogs. I saw some 125x125 banner ads in a few blog's sidebars, and the empty banner ads had a link to advertise on the site, and it looked as though it was a link built with some type of wordpress plug-in to work with those types of ads. Any ideas what this could be?
If you check out john chow's blog theres just been a free plugin released for this I use OIO Publisher though as I can set ads to be taken off after a time period and I can record how many clicks each ad has received.
that new plugin John Chow mentioned was developed by Unique Blog Design. I use it, it is really easy to use. You can find it on their website.