Richard,

Try rendering w/out GI/Radiosity using just Global Ambience and see what happens to the edge. My guess it will render fine with the lower settings. Of course it won't look right, but it's a quick test to see if the AA issues have to do with the Radiosity settings. If so, then there are a few settings you can use which might help for small surfaces like that. Let me know and I'll dig around and see what I can find.

-C