I'm from Suffolk, though my parents are not, so I never developed an accent per se - like Mr Tea, I'd consider mine to just be a generic southern accent. (You know - 'grarse', 'barth' - elongated vowels and, I suppose, leaning more towards RP than not.) The Suffolk accent is incredible, though, it can be almost impenetrable when speaking to 5th or 6th generation Suf-folks. There's all manner of curious dialect, too, which I've never encountered elsewhere (vastly different from, say, Norfolk and Essex, which are both right next door.) "Mahn't no" for "must not", things like that. I like it, though it doesn't confer an air of intelligence on the speaker.
Subconsciously or otherwise, people definitely favour certain accents over others. Loads of call centres use Scots or Geordies because these are, supposedly, soothing, trustworthy voices. (I can't find a link to back this up, but I'm certain I've heard this espoused before - QI, perhaps?) By contrast, you rarely hear Brummie voices anywhere, in the media, advertising, radio, whatever. Does that constitute accent-ism? I suppose it does, but then I'm pretty sure accents are tied up in other regional prejudices so I don't know if it's a demonstrable phenomenon in itself.
Looking at your original post, gumdrops, I'd rather in some ways that people who had had the benefits of an expensive education did stick to their guns and keep the plumminess in their voices - far worse for me is when they try to 'estuarise' their accent and vocabulary in order to disguise their privileged upbringing. Keep it real, you fuckers, even if that does mean being hoity-toity. I've been to clubs where there are various fucking Henriettas and Arabellas and sundry other double-barrels loudly exclaiming, "Oh my gohhhd, this is so sick, isn't it, girls?" whilst taking pictures of each other to put on Facebook. I silently fume and 'accidentally' tread on their feet in my grubby trainers. I've no problem with the adoption of slang, but it should be the slang of your social group, not another's - there are some words that just don't suit certain accents.