Who's putting up all that cash?
IANAE, but I'd guess it's Bitcoin bigwigs who cashed out at 20-30$ and aren't really in a hurry to buy.
And why are they keeping it in Mt. Gox?
It takes ages to withdraw, so they may as well play with the market. They could push up the price or just risk buying coins at 1/2 the price they sold at.
I just want to add something to that.
This is one potential scenario I use, and in my mind, it is equivalent to an imminent crash. If the money influx dropped significantly, take even the current (low!) volume, those orders would be filled in a matter of two days. It would cause a classic crash, setting in within the next week or so.
So, if you hold on to BTC, you better hope it's not just early adopters playing speculator who kept the price supported for multiple days.
Most likely, it's a mixture -- and the proportion will determine the short-term outcome. I guess we'll know more by the end of the week.