American pop music has typically conformed to a more or less standard accent, too, and it is not the so-called "General American" accent (supposedly closest to the old Midwest accent). In fact, it has more of a Southern aspect to it -- perhaps reflecting the history of our popular music. Really, it is most notably an African-American accent (which has aspects of Southern accents). The majority of American pop music, until recently, can be seen as an outgrowth of Southern genres, be they "country" or "blues." The most notable feature is the avoidance of "r" (nonrhotic pronunciation).

Though my own Northeastern regional American accent is more or less rhotic -- albeit occasionally r-less in certain contexts/registers -- I don't feel a bit of a phony singing songs without r. It's how songs are sung. People/I believe it sounds appropriate to singing. We are not stupid for thinking so.

When I hear a Californian sing a similar style, and given that a Californian typically has a much more present 'r', I am slightly amused. That is because the difference between his/her speaking and the singing accent is much greater (i.e. than I perceive mine to be). However, who am I to judge? Singing with Southern/AAVE tendencies is part of what makes a singer sound competent -- some would say "authentic." If you can't sing that way, you're booted off American Idol immediately!

I am intrigued by what I hear as a more recent phenomenon, and I don't know exactly what genres it falls into (though I have heard it in, say, the Emo that my nephew listens to)... where the accent it quite different. The 'R' is VERY present. It is like an Oregon/Northern California/Washington accent (I guess). It seems to convey something very different than the more mainstream/typical pop music. On one level, it is distinguishing itself as something new and alternative to the mainstream. However, in my very humble opinion, it is putting out a very deliberate "White" sound that contrasts with the usual "Black" sound. Whereas the "Black" sound has become the shared sound of pop amongst singers of all ethnicities, I can't help feeling that this newly-accented music is very oriented towards "White" listeners!

For UK accents I'd cite Street Punk / Oi! as a genre that has remained very true to local accents. You can't very well sing "England Belongs to Me" in a Yankee accent.

When I DJ to Jamaican music, my chat on the mic is necessarily in a sort of posh Jamaican accent of sorts. It is not in deep patois -- that would be a sort of overacting. However, there is a sort of accent shift that has emerged as a sort of "received pronunciation" for Jamaican music and which is appropriate for foreigners to use (see e.g. English sound man David Rodigan). What I find jarring -- but which has nonetheless gained widespread acceptance by the "natives" -- is the use of real, basolect creole by young foreigners doing Jamaican music. So long as you are respecting the culture, adopting the accent seems to have emerged as preferable to singing in an accent that is "foreign" to the genre.