I suppose it depends on what kind. "3rd person" covers a wider variety of possible control schemes than first-person does.
Over-the-shoulder 3rd person, for instance? Probably my least favorite gameplay style, period. Aiming and moving almost always come off feeling awkward and I end up abusing any first-person aiming options available, even if it's otherwise a hindrance (slower movement, only certain weapons use it, etc.). I think I'd honestly find games like Grand Theft Auto simpler to play if they used a FPS control scheme; of course then you'd miss out on hilarious ragdoll physics when you get blown up, but eh, they could always throw the camera out wider to show you getting blown across the interstate if need be.
More RPG-style games, on the other hand, seem more fun in 3rd-person since, well, I get to see the character, and I typically don't need to "aim" anyway, just target things and use abilities. The camera angle isn't a detriment to the gameplay.
If you just mean using the Source engine for 3rd-person gameplay, I don't know of Valve-created examples, but I'm pretty sure Alien Swarm and Vindictus both use the Source engine, don't they? If I'm remembering that right, then yeah, there's some good potential there in the engine. I'm just not sure what kind of game I'd want to see made with it from an exterior perspective. I'm not a physics-engine-buff, really. Did you have anything in particular in mind, Ink?
Sidenote: using one point of view isn't the same as doing only one thing. TF2, Portal, L4D and Half-life are all first-person shooters, but they're considerably different "things." That's not trying to be all MUST DEFEND VALVE - I only play one of those four games and kinda loathe one (TF2, neh) - that's just observing camera view isn't the entire game. There are other mechanics at work to make a game, too.