I doubt cold fusion was an elaborate hoax on their part. The two scientists, Martin Fleischmann and Stanley Pons, were probably 100% certain of their findings back in '89. But they made the mistake of being too enthusiastic. They didn't go through the proper scientific channels (with painstaking independent controls before publication) but went directly to media. Thus the whole thing blew up in their faces as a huge scandal.
On the other hand, it wasn't very clever to deny other researchers access to their labs afterwards. Sorts of implies guilt in a way doesn't it?
But I must say that I feel sorry for them as it turned out. Chances are (albeit it seems extremely improbable if you know the physics involved) that they were onto something but never got a chance to develop it further because of being over-enthusiastic.
Then again, that particular experiment was very simple- and was later extensvely tried by others- so one should think that someone should have stumbled onto their solution by now...
.
SlimJim