You can most certainly get USD on TradeHill. In fact, at this second MtGox is offering 14.01 for sellers, TH is offering 14.21, so you'd have incentive not to trade on MtGox at all if you wanted to switch over to TH.
Blah, blah, blah...
Yeah, those 20 BTC walls will make you some fast cash. Tradehill doesn't have the volume to make decent money. SORRY!