Would it be acceptable in an iPhone app to show the iAd underneath the bottom tab bar? I know that this is not ideal, but for programming reasons I find that it may be necessary. Does this impact the user experience in a significant way?
This is not only bad UX as Devin stated, this also (most likely) won't be accepted by Apple. So the best thing to do for you is ask on stackoverflow or any other coding site for some implementation help.
See the App Store Review Guidelines
Apps that artificially increase the number of impressions or click-throughs of ads will be rejected
Apps must comply with all terms and conditions explained in the Apple iOS Human Interface Guidelines
Apple and our customers place a high value on simple, refined, creative, well thought through interfaces. They take more work but are worth it. Apple sets a high bar. If your user interface is complex or less than very good, it may be rejected