Although I know Hitchcock and Dali worked together, I can't say Dali actually painted the mattes; he may have designed them for studio painters to copy.
You're right about the fake Dali paintings, though. I don't know all the ins and outs of the story, but when he died, I understand that authorities were faced with re-proofing every possible Dali work. Apparently there were hundreds of fakes. The story got quiet enough after that, that I don't even know how successful they were in sorting them out.
I think there is a book devoted to the whole controversy of Dali mass producing art (including signing blank paper for prints to be printed on rather than approving finished prints).
In other news, there was an excellent Dali book released in the last few years (and is now making its remaindered way to Half Price Books) about a fun house Dali designed for the 1939 World's Fair.