Blame it on the people who say “An historic event”.
Well, “An historic event” IS correct if you pronounce historic with a silent h.
Which nobody in North America does…
I think there are people in North America who say “historic” without the “h” sound. Mostly around Boston I think. I had a former boss from there who even said “Yooston” for “Houston,” although you could never pick up on any Bostonian accent in his speech.
Lots of people do. I do, and I’m from New York.
… whereas they all pronounce herb as “urb”, for no fathomable reason.
It is also worth mentioning that the pronunciation evolves over time. If you look at Edward Gibbons The Decline and fall of the Roman Empire you will find that he must have pronounced the words horse and usual differently, as he consistently writes an horse and an usual [xyz]. But then, he wrote more than two hundred years ago.
Don’t worry; even if it weren’t Friday, you’ve got us to laugh at you. 
[QUOTE=CalMeacham]
'cept, of course, if the vowel sound is a “long u” :
a unicorn
a unified field theory
a uintatherium
QUOTE]
Not quite. There’s an implied “y” sound at the beginning of these words, not written, but spoken. We actually say “yoonicorn”, “yoonified field theory”, etc. A long “u” is the sound of the vowel in “food” or “rude”. The “y” acts as a consonant and the article conforms accordingly.
Watch out or they’ll send Noam Chomsky after you with a big stick.
As I do, you probably remember your teachers in early childhood telling you that long vowels sounded like the name of the letter, while short vowels sounded like something else. The reason your linquist friends get so worked up over this is that vowel length means something quite different in phonetic terminology. It is exactly that…how long the vowel is held before moving on, as it were, to the next sound. For the most part, true vowel length in English is determined by whether the following consonant is voiced or not. If the vowel is terminal, then it is long. For instance, try pronouncing these words to yourself:
bit bid
bet bed
but bud
Most people will pronounce the vowel perceptibly longer in ‘bid’,‘bed’, and ‘bud’, the ‘d’ being the voiced analog of ‘t’.