There are still many people holding large amounts of bitcoin (because they mined them in the early days) and just one of them selling everything would cause a pretty large downswing.
As volume picks up (market depth has nearly tripled in 2 months) this is a lesser problem.
I found https://bitcointalk.org/index.php?PHPSESSID=h72gmr1luhb169j1...
which shows that the top two wallets are gone now and the 80,000BTC Wallet has grown to 111111.11
The top 400 goes down to about 3,000BTC. A bunch of those could be the former largest ones broken up.
http://www.zerohedge.com/news/2013-03-10/demographics-bitcoi...
I read somewhere (source unknown, possibly unreliable) that 75% of all BTC are hoarded (ie. not in circulation).