I can't remember where I first hear the rule, but one cite:

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