This has puzzled me all along; was Wilson suppose to be able to claim/infer that Cheney sent him and not be contradicted because of who his wife was?
Right. Either the White House, which certainly has a right to poke holes in Wilson's patently false statements, must tacitly corroborate the falsehood that Cheney sent him, or must "out" Valerie Plame and pay the price. A Hobson's Choice.
It would have been infuriatingly dirty pool for Wilson to purposely set that up. He's certainly capable of it, but I've always thought it wasn't intentional entrapment, but rather that Wilson blundered into the situation, and then the Clinton machine helped them construct that beautiful Catch-22 out of it.