You can ask Fallout: New Vegas Lead Designer and Project Director Josh Sawyer anything you want (except about Fallout or Bethesda) via his new Formspring page. One recent question asks:
Which perspective do you prefer, as a designer and as a gamer respectively: isometric, third- or first-person? Is there any source proving a common belief that FPP is more immersive?
I doubt anyone's actually done a study on gaming perspectives and perceived levels of immersion. I think it's just that: a common belief.
For me, it depends on what you/the designer is trying to accomplish with the camera perspective. For a lot of games with tactical combat, an "iso" perspective makes the most sense because you want the player to be able to see and process a lot of information about the environment and combatants. The same applies to strategy games that span large areas (or don't really focus on a single character).
Close third-person cameras can work when you want the player character (often alone) to navigate the environment with climbing/platforming elements or when real-time melee combat is central to game play. I think it can also work well for some horror games (e.g. Resident Evil 4, 5, Dead Space) because it combines a limited view of the world (anxiety-inducing) with a close-up picture of the player character's vulnerability.
First person is probably the easiest to work with, but it's not useful for all types of game play. If a designer wants to effectively remove the player character as an entity and focus more heavily on the environment, first person is the way to go. Personally, I lose all sense of character in most first person games and that's usually a big negative for me.
Interesting. Go and ask him some questions of your own.