In addition to all the other vowel shifts I mentioned, there's the change from a short E to a short A. Years ago there was a commercial wherein a blonde lady - playing a stereotypical blonde, of course - finding that she couldn't pronounce DiGiorno, said, "It's Franch." There was a song even before that which included the line, "We were only frashmen." And just this morning (August 23, 2012) a local TV station talking head spoke of an "adge" where I would have said "edge."
I've laughed to myself at the way Brits don't pronounce words according to their spelling (e.g. "Americer," "med'cine"), but it's gotten to where a whole host of Americans don't pronounce words according to their spelling either.