I fully expect that owners of large caches of bitcoins will sell them privately and discretely to various investors that wish to not use financial exchanges like Dwolla, Mt Gox, and Tradehill.
actually when i was moving a decent chunk of change out of tradehill a while back(love no withdrawal limits!) i received an email from them asking if i wanted them to setup some direct trading with some high rollers so I guess the exchanges are also mediating situations like that as well.