My reaction to this depends upon what you told him when you hired him.
If you hired him to do front end development, then, yeah, he's over the line.
If you tell someone that they're going to be working with the chief architect on figuring out the networking substructure and then throw him in with the web monkeys with no advance warning, they have a right to be concerned.
It also depends upon the size of the company. At a big company, I would give zero slack on this--this kind of shenanigan is indicative of a political battle and you need to hold your ground.
At a startup, I'm going to cut you slack if I see everybody is busy and you say "Look, we need you move this pile of crap. Sorry. Someone has to do it, and everyone else is busy. Interesting thing X is coming down the pipe, but right now you are two idle hands. Grab a shovel."
