Is that actually within the rules? I'm skeptical.
You can be as sceptical as you like. Nevertheless I did it, and it seemed well within the rules to me.
What happens when PCs start trying to circumvent the combat rules for other monsters.
The PCs don't circumvent any rules.
So I guess the question is, "What happens when the players want to frame a confrontation as a skill challenge rather than a combat, for resolution purposes?" The answer to that question is - that can be sorted out on a case-by-case basis. Most of the time it is not going to come up, because it is obvious what is within the scope of the skill and skill-challenge mechanics, and what is within the scope of the combat mechanics.
In cases of disagreement, being D&D, it is ultimately the GM who is in charge of establishing the mechanical framing (along with the fictional framing).
In other words, there are three sets of rules. There's reality, there's the PHB, and then there's your shared mindset. Which begs two questions: one, what if you try playing with someone who doesn't just intuitively agree with your conception of the world, and two, if you already have that shared understanding, what are the game rules really adding to the mix?
Neither reality nor shared genre conceptions are rules. So I would say there is one set of rules - the game mechanics - plus there are other considerations that actually guide the creation of the shared fiction.
Two of the players in my 4e group I hadn't played D&D with for about 10 years prior, and even then only a handful of sessions. No clashes of expectations have arisen. I think this is because the 4e rulebooks do a pretty good job of establishing the basic foundation of play, for instance with the discussion of tiers (reference upthread), the descriptions of paragon paths and epic destinies, the default cosmological backstory, etc. Points of detail get worked out along the way - typically, if a player wants his/her PC to do something my approach is to say yes and set an appropriate DC for an appropriate skill check.
As a general rule, the game is not going to break because a player's PC got to do what the player wanted, and the game has pretty clear rules for setting DCs and for adjudicating the consequences (both good and bad) of checks that are made.