Are there any dialects in English that are unintelligible to other English speakers of other dialects?
2. Are there any English dialects?

Definitely. Only last night I played for my partner (a native speaker of California English) a snippet of this clip featuring a native speaker of Newfoundland English. (His bit starts at 0:48.) My partner could recognise a few words, but he had no idea what the man was talking about.

What done it? I think he means what did it. I can hardly understand him lol.

No, he means "What done it?" The rules for forming the preterite are different in Newfie dialect, just as surely as they are different in Swedish from how they are in Norwegian.

Preterite? Myaw? Qu'est-ce que c'est que ça? I'll have to look that up.
Edit: I just looked it up, and it's just the simple past. Thanks for being confusing, linguoboy. He does mean "what did it" -- translated into my dialect, that is.

Confusing? Preterite has been established grammatical terminology for two thousand years. Its use to designate the English synthetic past tense goes back to at least to the 16th century.

Easier said than mountain passes.

im from ohio but i live in the south.

"all y'all come 'ere!"

Sitha here laddies, een tha's whafflin' o' Inglish daalects then t' Tyke speak i' a reet grand un tae be beginnin' wi, di tha ken? Tha's nor sae wick wi' t' Yorksher een tha' cannae understan' this thassen! Nay, lad!

See now, Newfie and Scottish accents I don't have much trouble understanding. Maybe that's because I've got a lot of exposure, I dunno. But you know who does have a really distinct dialect, which is often unintelligible to local folk? Indians.

It's not that we can't understand them, per se... Most of their grammar is the same, though there are some minor differences. The problem is, really, that some of them speak so goddamn fast, that with their accent it still sounds like they're speaking Punjabi, or Hindi (Or whatever their mother tongue may be). To us English natives, it's like a whole bunch of syllables crammed together. Heck, I'd say it's faster than when those TV presenter guys read out the FBI warnings aloud.

And then there's Cockney English. It's not that their accent is particularly difficult to understand, even if it is strong, but there's a whole lot of vernacular going on there from a combination of both Cockney Rhyming Slang, and historical Thieves' Cant.

"Will ye be eatin' wi' Uncle Fred and Johnny Rutter?" (Would you like bread and butter?)