Something similar happened to David Tennant during the DW Series 3 episode "42." As he's yelling over the intercom for someone to bring him a spacesuit, he shouts "Just get down here!", and his Scottish burr is very noticeable to me.

As an American, I'm actually quite impressed by some of the British and Australian actors on US television who manage very decent American accents (Hugh Laurie, Damian Lewis, Rufus Sewell, Simon Baker). For some reason, Brits and Australians seem to have an easier time with American accents than Americans do with British or Australian accents. Heck, even Karl Urban pulled off a decent Southern accent when he played the younger version of Tommy Lee Jones' character from the "Lonesome Dove" series, which gave me much more confidence in his casting as Dr. McCoy for the upcoming Star Trek movie.

"Although Simon Baker spoils it by going all Aussie for the voice-over on the "next episode" bit at the end."

Yeah, it is a bit jarring, but his accent in the show is good. I've liked him since LA Confidential.

"Jonny Lee Miller says it's something to do with how much effort needs to get put into the mouth movements: he says that English people need to put more effort into it, so when they try an American accent they can be lazier, whereas it's harder for Americans to go the other way. Plus, I'm sure exposure to the accents has something to do with it."

Ooh, I forgot about Jonny Lee Miller! I really liked Eli Stone and was upset that they canceled it. He was quite good in that, and his accent was great.

I don't know about mouth movements (although, he's an actor who does more than one accent, so I'm sure he's on to something there), but speaking with an English accent seems to require more precision than speaking with an American accent. Brits tend to enunciate more clearly than Americans; we have a tendency to swallow our consonants I think.

I think you're being a little harsh on the 'Englishness' of his accent (and it's not The Devil's Whore accent which was... well... not fabulous). Anyway, I'm always willing to cut actors some slack when they're in high emotion/anger mode since that's when its hardest to hold onto a non-natural accent. That so many are pretty darn good even in those situations is impressive.

Because if I hear one more poor Dick van Dyke English accent on US programmes I feel i may scream.

"It's all right. I knew it was you. You could always LiveJournal up and then you'd get your piccie as well as a greeting."

Yeah, but the corporate IT wanks block LiveJournal here at work I can do it at home, though.