啊，明白了 - 我必须添加以下标志(无论目标操作系统版本如何)

Ah, got it - I have to add the following flags (regardless of targeted OS version)

 -gcc_flags "-weak_framework iAd"

这一切都在 MonoTouch 3.2.6 BTW 中.我还没有在 4.x 中尝试过，但是当我尝试时我会更新这个问题.

This is all in MonoTouch 3.2.6 BTW. I haven't tried it in 4.x yet, but when I do I will update this issue.