I GM a fair amount of on-line (Skype & Maptools) D&D4e games (52 a year) and about half that around the table- I've used the game in classes (mostly with Electronic Game Design students). I've noted all of the points made so far- players that RP all the time, players that need cajoling to RP, players that require intense (immersive) dramatic action to get them to RP, players that require some kind of reward to RP etc.
The bottom line in my IMHO (and experience) is that the players that have more out-going personalities (and less hang-ups about acting out a little) sometimes (more-often) make good RP'ers (as in they try more often), require few (or fewer) rewards and... those that are naturally quiet and or a little more self-conscious don't (or do less). I don't mean don't altogether- they try at times but... are happier to let others carry the narrative (or they think they are better suited to offering advice).
In my regular games the players (none of whom knew each other before I came in to their lives) have very obvious boundaries- which are fairly unbreakable, or at least in the years I have been gaming with them they've not changed their behaviour much. That's not say I haven't tried to change them, to wring more RP out of them, noticeably the players that found RP easy at the start are the players that continue to find it easy to do as the game progresses (and vice-versa).
I'm not saying the quiet kid doesn't suddenly adopt a persona, talk in character all the time and address his imaginary horse- it's just that I've observed the above a fair amount of the times from the noisier, more outgoing members of the group.
Personally I do voices, have been known to sing, shout, scream and protest my undying love for inset PC name here (when I'm playing the princess of course- although our last princess turned out to be an Aboleth in disguise) at the drop of a hat. I leave plenty of space for the players of course (it hurts my throat after a little while- I've got about 20-30 up to 1 minutes repetoires).
RP is great, particularly good RP- even bad RP tends to move the narrative on (more so if you have a good GM who can direct traffic).
Combat is great, particularly combat with a direct baring on... something (beyond PC death), rescue the whatnot, save the thingummy, or else you're against the clock, or in a fantastical place. But sometimes (often) because of the less immersive elements it leads to RP drop off- just an observable IMHO.
It's particularly difficult in the opening games when players are still learning the rules and things have to slow to be checked. In classes (with my students) I go very rules light to get things done and enncourage them to RP- 'anything goes' is the rule in these games- most often the thing that goes is the narrative/adventure- but hey-ho.
My example would be, we have a new player in my on-line game, never played D&D4e before, he's playing an existing character (until he decides what he wants to play). First session he talks in character- something many of the other players (who have been in-game for 2.5 years) have not got round to yet (third person- no problems). In combat he doesn't name the powers he's using, he tells me what's going on- and makes it up, he asks (actually he didn't ask), he stated- 'I reach down and grab the Famine Hound, whirl it around my head and then slam it in to the other Famine Hound', then he delivers a slick one-liner in character (or two) and asks what dice he has to roll, and could he make an Athletics check as a Minor to help with the move.
We call this 'stunting' in our game- I didn't tell him that the rule for this existed, and he didn't ask- he was just there with the his PC and there were two Dogs and he wanted to hit both of them (he's a Fighter Brawler). One of the other players (on maptools) is content to just roll Skill Checks with no conversation, every week I say to him- don't just roll the dice, tell me what you're doing- describe it, explain... and he mostly does (and remembers to describe the action the next time, but not the time after, or the time after that- as I continue to watch die rolls flash by on Maptools). And a majority of the guys in game are speaking English, their second or in some cases third language.
All my players are about the same age (for both games), similar(-ish) backgrounds (qualifications, families and okay jobs), the ones talk a lot tend to do so in-character, and get immersed in the game. The ones that don't... well, don't.
They accepted it. It simply didn't change their behavior any.
