Although the bogey-man de jour for the radical right and the McCain campaign seems to have shifted from William Ayers to ACORN, if you are looking for information to debunk the Obama-Ayers links, try this article from Politifact.com.
We especially like this quote from the article:
The campaign appears to have confused two different definitions of the word "radical." Clearly the invitation referred to "a considerable departure from the usual or traditional," rather than "advocating extreme measures to retain or restore a political state of affairs."
So not only are McCain's campaigners making unfounded attacks on Obama, but they aren't too bright.