DAO token trading started on May 28th. Since then, DAO token have been steady between 0.00022 and 0.00023 Bitcoin per DAO.
As of 2016-06-07, here is the list of markets where you will be able to trade DAO token:
It should be noted that with The DAO Tokens specifically, you cannot send the tokens if they are currently "tied up" in a vote. This prevents a single token from voting more than once.