Hi eservice,
Thank you for your comments.
Randomization is controlled by the sql query rather than using php to generate a random order. As the number of ads for a zone increase, the "distribution" does become more even.
I had not planned on incorporating a "timed" rotation for ads. To do so would require an ajax request triggered by the user-defined expiration setting. To further expand on this functionality, each ad could have a different "exposure" setting. It is not that this functionality cannot be incorporated rather, at first glance, it would introduce a level of complexity for the user that I think out weigh its benefit. I will, however, add this to my 'research' list to see how easily I can incorporate and document this functionality.
Thanks again for your comments!
~Coyote