Not only is Dominic West attempting an American accent, but it's supposed to be a Baltimore accent - for my money the strangest local inflection in the US. It's probably best represented on the show by Prop Joe and Lt. Mello, who are Baltimore natives, though McNulty does a pretty good job.
To guiltpuppy and JG, if she's still reading - all I can say is that the show's only real fault is that it's difficult to get people to watch it. It's easy to be sucked in by cliffhanger dramas like Lost and 24 in an episode or two, but The Wire took me at least half a season to begin to figure out which characters were which - and what the hell they were saying. But, yeah, obviously way worth it.
I don't walk it like I talk it cause I run it