Wikipedia says the name was originally "Magic: The Gathering Online Exchange" and cites The Verge.
The Verge says the same and cites... Hacker News.
I still believe it, but circular citation is often cause to raise an eyebrow or two!
EDIT: Note the mountain image on the about page. https://mtgox.com/about-us
I have only ever heard it pronounced correctly twice, people typically say it as "fortay". This has now become so common that some dictionaries are showing it as a valid pronunciation.
Language is always evolving, which is great.
Your belief about which is the "correct" pronunciation depends on whether you believe the "correct" etymology is French or Italian. It appears to me that people agree that the word came into English from French earlier, but also that (at least in 'murica) people have settled on the other pronunciation.
It's not that there's a correct pronunciation that got corrupted, as you suggest. There are two legitimate derivations of this word with different pronunciations and one has won out.
By the way, the word in French would be pronounced \ˈfȯt\, which is a common usage in Britain. And it refers not to a fort (which is the Italian etymology for "a strong point") but rather to the part of a sword between the middle and the hilt.
Source: I like to read about usage and etymology. Sadly my dictionary of classical word origins suggests that forte comes from fortis in Latin, which (while ultimately true) doesn't reflect either later usage or pronunciation.
The correct pronunciation of "forte" as in an area of strength or speciality is "fort". Older dictionaries will give this as the correct pronunciation but more modern and especially online dictionaries sometimes give "fortay" as an alternate pronunciation.
Although I think (in the spirit of pedantry) it's worth noting that the french pronunciation of the word depends on whether it's fort or forte and, of course, the first syllable of the next word!
The events on mtgox in the last few days are irritating but not all that surprising. Bitcoin as a protocol is still solid but traders need to take their business to a less amateurishly run platform. Mtgox already has several competitors (Bitstamp seem good) but a shock like this was required to overcome the critical mass they had.
As an aside - it's fascinating to watch a new financial system being built from the ground up. Through the regular disasters in the Bitcoin ecosystem I'm starting to see why parts of the 'real' banking world function the way they do.
If anything, MtGox should be congratulated for executing a wildly successful pivot.
Descriptivistically, the "Mount Gox" backronym seems to have more currency. (pun intended, but immediately regretted!)
Right at the start she clearly says "Mount Gox" - http://vimeo.com/m/40056755
Mt. Gox ~ Fort Knox
A bank or brokerage should have a name of weight and substance, to give customers a sense of security.