Slate has an interesting little article on the Northern Cities dialect and the vowel shift going on there. (Though seriously, I doubt it’s entirely a recent thing, because there are some historical literary references to similar sounds.)
Almost all dialects tend to have vowel shifts, though. That’s a big part of what you instinctively imitate when you imitate an accent. Likewise, most dialect speakers don’t recognize themselves as having an accent of any kind. It’s other people from over there who talk funny, not you. These things lead to confusion and to expressions that clarify what you mean. (For example, dialects where “pen” and “pin” sound similar have expressions like “ink pen.”)
What makes it humorous (and which Slate doesn’t actually come out and say) is that a lot of folks from the North really do think that they have no accent, or the standard American accent; while simultaneously, they think anyone with an accent is stupid and ignorant. (For example, the ridicule for Bush’s non-standard but normal pronunciation of “nuclear.”) So now you can point and laugh at the Northern cityfolk, if you want.
But maybe people will start to be a bit more mature, and smile at each others’ foibles of language without mocking them. Language is fun and beautiful, one of our greatest technologies and arts. Its multiplicity of manifestations, and the stretchiness of our abilities to understand and use it, are part of its charm.