"I mined about 13000 bitcoins in 2010 when it was possible to mine with a single quad core CPU. It was worth about $100 when I stopped mining in August 2010, and I was ecstatic that I had earned a nice steak dinner for doing absolutely nothing. After I missed the first peak at $31 in 2011, I was determined to sell everything if it ever hit that price again. Unfortunately, little did I know the price would surge to $145, and I sold most of my stash at the then all-time high of $40-50. Not quite a bitcoin millionaire, but still a good chunk of change (of which a sizable amount will be going to the IRS)."
The trick is to not get greedy, start taking profits as the market rises and then put those profits into less volatile investments.
A guy used MtGox to sell 5,000 BTC the other day, and caused a small dip - about 15%, if I recall. That's about 500k USD out in an afternoon.
Some exchanges also have "dark pools", designed to allow big investor to get into and out of the market "quietly".
Realistically, if you had that much BTC to sell it would be wisest to do it in phases, or better yet, tiers. Offer volumes of BTC at various "outrageous" price levels (eg, $5 over market, $10 over market) and make sure you put a substantial amount of volume at each price level. Whenever the market surges you'll cap the exchange rates at your price levels until all your volume is depleted. In fact, you can see people already doing this on some exchanges...
There must be a way. "Man Tries to Sell House for Bitcoins" [1], how is he ever going to get his money back?!
These exchanges will convert your currency to/from BTC