From what I understand as long as they are not excessive it's ok. For example some Ad Networks have "pop-unders" which only serve themselves to the same user once every 12 hours. Fastclick is one company. Click the link in my signature.
then i have to pose this question...wouldn't this pop up drive be "illegal"? I really don't know just asking....but since that question was just asked it got me thinking....
Answered my own question After looking at their policy it states: The Ad Unit must be shown in the requested page, not as a pop-up or pop-under window or in an e-mail