Here is an argument. If ought implies can, then cannot implies not ought. Not ought implies either permission or ought not. But permission implies can, since one cannot permit what is impossible. So cannot implies ought not. If can is a descriptive term, then this shows how to derive an ought from an is.
[Update from August 21]
Wesley Buckwalter brought it to my attention that, actually, very few subjects will agree that moral ought implies moral can. This appears to be reason to dispute the argument above.
I think part of my conviction that ‘ought implies can’ has been undermined in part because there is an ambiguity to the word ‘ought’. It seems to me that ‘ought’ implies either ‘should’ or ‘must’. While ‘should’ only implies ‘sometimes can’, ‘must’ implies ‘can’ in some broader sense. And the moralist tends to be more interested in imperative-like statements (e.g., musts over shoulds).