Actually it isn't the cyclorama, but the AO which comes with IBL (IMHO). When using AO you get the same problem with any objects having big flat surfaces (though I have to admit I newer saw it that strong!..) For instance I get it all the time on Stonemason's sceneries. Poser bug.

To fix it, raise the Shadow Biais, and check by making local renders on the problems zone(s) till the problem dissapears. AO will look a little less good, but short of rendering 2 versions and patching problem zones in Postwork there is no know (to me) solution.