Do you think that 500,000 BTC can be obtained from thin air?
Under the assumption that it really belongs to a single user:

• If they were acquired when Bitcoins where worth 0.01, it would still had been a significant investment: $5000 USD
• If he didn't buy it and farmed it, the farming of 500,000 BTC signifies that he knows the intrinsic value of it, so he was quite dedicated on the project.

Both scenarios make very implausible that the owner would suddenly forget about their generated/purchased bitcoins, it shows interest, dedication, appreciation and/or faith for the bitcoin economy.
In the event of forgetting about it for a while, that hypothetical person wouldn't neglect that the prices increased more than 200,000% from the time he acquired/generated them.
And certainly everyone who is marginally related to bitcoins must have heard about jumping to $30 USD = 1 BTC. IF he had forgotten, that news might have reminded him that he was awesomly rich. Such news sparks interest again towards the bitcoins.

Who ever was/is the hypothetical owner, MUST HAVE KNOWN about his WEALTH.
Considering all above, the "Ignored and abandoned" argument is highly, very highly implausible.

Now, who would put more than 500,000 BTC in one exchange? That really escapes me.
Anyone who has that much in one site must have close relationships with the owner, otherwise I don't see how you would trust such amount in one place.
On the other hand, it would also make sense that it is a government conspiration, but considering that the bitcoin economy is still experimental I wonder if they would take such preventive measures to try to destroy it.

Personally I rather the explanation of the cleaning lady cleaning the keyboard and pressing "Sell 0.01"... would you admit such error if this were true and you were MtGox? LOL