I'm goin' dahntahn tomorrow to catch me a bus
Having been in Pittsburgh for the last six weeks, I can attest to the reality of the pronunciation of "downtown" as "dahntahn". I heard it when I first got on a bus and asked, "Is this bus going downtown?" and the driver replied, "Yeah, dahntahn." It was like a lightbulb going off in my head, bringing back memories of Linguistics 109 ("I'm goin' dahntahn to get me a sammich"). I can't say I've noted too many other Pittsburgh quirks though - didn't notice any sammiches, for instance. Being in the university area, most people I hear are decidedly not native Pittsburghers.
I'll try to pay more attention this weekend as I travel into the Northern Cities Vowel Shift area, but seeing as I lived in upstate New York for four years, the accent around there seems pretty normal to me now.