I was born and mostly raised in Michigan, and moved the summer before I turned 13 to Appalachia... Kentucky, to be specific.
My accent will swiftly shift to reflect the accent of the person I am talking to, be it a northerner or a southerner. I can talk like the biggest hick ya'll will ever see (complete with appropriate "southern" slang!), or with the cultured, eloquent precision of a newscaster. The vocabulary used shifts along with the accent, as well.
There is a second instance in which it will shift, and that's tied to my mood. If I am growing irritated, it will shift south automatically, no matter who I'm talking to. If I'm so angry that I'm about to bring the pain (psychologically or physically, it doesn't matter which) it shifts north, no matter who I am speaking with.