During my submission process for my latest WP7 app I found a bug that pubCenter verified:

“This is a bug that is now being addressed. You will not be able to receive text ads if your height is set below 70.  Thank you for bringing this to us.
I have no timeline as to when the bug will be fixed and the next update which will include the fix. I expect it will be fairly soon.”

I don’t remember setting my height to < 70 but either way its a good thing to be aware of going forward.