Hmm, I wonder what it is then - I can flip back to FF or Opera and the ads appear as normal - but not in the new IE7. I'm going to play with the security settings and see what happens, but at first glance it appears to use what I had it set at on IE6 and that worked fine in regards to displaying adsense.
There's an answer but what is it? Beyond that, I hope this doesn't become something that happens to lots of people as they start installing it - that could be very bad for revenue.
EDIT = A google search shows that the earlier beta version did have this happen to some people. So I am not alone, but this is not the beta version I am using.
Hmmm