Then again, "Common Errors in English", which I find pretty authoritative, says there's no difference. So maybe it's regional? Not sure. But it seems a logical distinction to me; for example, see the following headline, which would have a totally different meaning if you substituted "cannot" for "can not":
Re: Curious.
http://www.askoxford.com/asktheexperts/faq/aboutspelling/cannot
Then again, "Common Errors in English", which I find pretty authoritative, says there's no difference. So maybe it's regional? Not sure. But it seems a logical distinction to me; for example, see the following headline, which would have a totally different meaning if you substituted "cannot" for "can not":
Court weighs if doctors can not treat lesbian