Generally, wherever I travel, people assume I'm from there. That's because 1) I don't have much, if any, of an accent and 2) I have this weird habit of mimicking the person that I'm talking to. I'm in New York, I'll drop a "cuz", out west, I'll say "dude" all the time, down south, a lot more "y'all"s (although I use that one a lot just because it's useful to have a plural second person).
But I've noticed
something two somethings that give me away.
First, I drop syllables. More accurately, I combine them. I can think of at least two words where I do this consistently: comfortable and interesting. I pronounce interesting as in-trest-ing, as opposed to the more common in-ter-est-ing. This one doesn't usually get picked up by other people.
However, comfortable is a totally different ball of wax. I remember once that I had an interview all but laugh at me because of the way I pronounce it; actually, I think it was my pronunciation of uncomfortable that had him in hysterics. I gather that most people pronounce it as com-fort-a-ble. Well, for me, it's just three syllables: cumft-er-ble. Not only do I drop sounds, I transpose the order of the t and the r sounds. Un-cumft-er-ble.
It's not just the of-ten/of-fen difference; this is something that seems to me to be unique to the Midwest: http://www.antimoon.com/forum/t936-0.htm. Looking through the site after googling /interesting comfortable pronounce/, I came upon this thread: http://www.antimoon.com/forum/t9557.htm.
This leads me into the second point: well, it turns out that I know I conflate the pronunciations of "pin" and "pen". If you look on this map, you can see that the conflation just gets out of the south into southern Illinois (and the Bakersfield area of California, strangely).
Taking the INNER JOIN (look, a SQL joke) of the two areas, I think that it's really interesting that you can pinpoint where I'm from. Now, I do say wa-ter, and not war-ter, but I did grow up saying "soda" instead of "pop" (although I've since switched since I went to IMSA).
Here's a link that will kill an entire day if you go through the links: http://en.wikipedia.org/wiki/List_of_dialects_of_the_English_language