It's not really $500,000 though is it? As soon as you exchange a small amount of that into dollars you crash the exchange and the value drops significantly. I'd estimate it at about $30,000 worth.
On June 12th, $3,5M were exchanged on MtGox. For 8 of the last 10 days, the daily volume has been above $1M. I think that you could get these $500K in small chunks in a few days, provided you do it full time.
Without an unbiased third party auditing of their accounts I would be loath to place too much trust in data they themselves produce to say how awesome they are. Especially with such a significant amount of capital allegedly at stake and nothing to lose for bending the truth.
Granted the market is easy to manipulate with small transactions that can start large swings; as well it would be difficult to find a buyer if you wanted to liquidate a lump sum of this size without a solid discount.
That said, I think you're overstating the impact this would have. The recent downswing from ~$30 to ~$10 in a period of a couple days was speculated to be due to MtGox consolidating 20 times this amount.
Without knowing how much real backing the exchanges have it's hard to get a figure on what effect a large dump would have on the market. Additionally, you'll get shock dumpers who do a run on the exchange once they see value erosion. $30,000, although fabricated, feels right. Although i'd add that's probably an upper bound rather than expected value.
On June 12th, $3,5M were exchanged on MtGox. For 8 of the last 10 days, the daily volume has been above $1M. I think that you could get these $500K in small chunks in a few days, provided you do it full time.
Edit -- Source: http://bitcoincharts.com/charts/mtgoxUSD#rg30zigDailyzvzcvzl...