I think your concern is not misplaced. Back in September, I switched money from a money market fund that had somewhere between 8-20% exposure to mortgage backed securities and/or collateralized debt obligations, to a money market fund that was solely invested in US Treasuries (the Vanguard fund mentioned in a reply above). The New York Times article below explains the risk that normally "safe" money market funds are facing and that many will likely require bail-outs to prevent them from "breaking the buck".
