I thought I'd save anyone else the headache if they run across this one. I had an older app stop serving ads because the admob sdk was too old, but when I rebuilt it, the ads still wouldn't show up. After much banging of the head on my desk, I finally figured out that having the graphics compatibility mode for the old way of doing graphics turned on was somehow causing a problem with the ads. I assume it had something to do with the ad container.
Anyway, in case anyone else has older apps serving ads and comes across the same problem: update the app to be compatible with current graphic standards.