Author's take on it
Excerpt of the author's notes from Durarara!!:
It's an extremely bizarre title, but once you read the entire book... you probably won't get the meaning. I was close to finishing writing this book, and when the chief editor told me, "it's almost about time you gave an official title to the PR department", I said automatically,
In response to this string of syllables came the chief editor's response.
"Ho, I love titles like these with unclear meanings. Let's go with this... but how to you spell it in English?"
I didn't think it'd get approved, and while I was still dazed by it all, the chief editor asked,
"...Will you add on a '!' like in 'Baccano!' and 'BoWWow!'?"
Never expecting it to get approved and my thoughts in utter disarray, I said without thinking,
"It's rare we get the chance to, so let's add two exclaimation marks while we're at it."
After a moment's silence, I heard what I thought was the sound of rustling paper, then came the explosive laughter from the chief editor.
"Wahahahahahaha! It looks extremely stupid written down on paper! It's rare we get the chance so let's go with this!"
And so the name "デュラララ!!" was born--- but I'm still not quite sure what it means.
 Translator's input
I keep changing my mind. Will leave it as 'Durarara' after all.
[s]EDIT: well, caving in at long last. DRRR!! it is, seeing as it's named as such in both the anime and the manga.
That was informative. Anyway, I choose to romanise it as 'Dulalala!!' because it links to the word dullahan (hint: it's got to do with the main character). Also, the publisher's website URL uses this spelling as well.
I'm aware that the anime production studio and lots of fans choose "DRRR!", but I can't for my life figure out why. Will get back when I do find out the reason (maybe it lies in v4 or v5, but I don't have either).[/s]
 List of romanisations out there