Wasn't 45K BTC about what was stole from Bitcoinica? Maybe this is the tactic Zhou took to fill his reserves. If that's the case, you'd expect that wall to hold.
Alternative theory: Someone put up a blatant wall to define a temporary bottom and is selling higher, somewhere around 4.8 USD per BTC, using a bot to avoid placing a wall there. I'm not sure the volume we're seeing really justifies this theory. Does anyone have historical depth data for the last few days? If this scenario is correct, you should see significant resistance at 4.8-ish which isn't reflected in the market depth charts.