Such funds are sold to investors as the equivalent of cash, and their $1-a-share net asset value is considered inviolate. But if the funds experienced big losses, the value of the assets could be vulnerable.
The problem is that some of these funds hold commercial paper, and according to the article:
Standard & Poor's, the ratings agency, warned yesterday that it might downgrade several issuers of commercial paper, a short-term I.O.U. by companies that promise to repay loans typically within a few weeks to a year. In these cases, S.& P. said, the commercial paper was backed by residential mortgages.
It should be noted that money market funds have many safety precautions in place to protect the investors and have a long history of principal preservation. Also, it's widely believed that the money market funds offered by major brokerages like Fidelity and Vanguard provide extra safety since these major brokerages wouldn't allow their money market funds to "break the buck". Nevertheless, there are no guarantees or FDIC insurance.
It's important to note that money market funds are different than money market accounts. Money market accounts are offered by banks and credit unions and are typically federally insured by the FDIC (for banks) or NCUA (for credit unions). Money market accounts are essentially the same as savings accounts. Unlike savings accounts, they usually have limited check writing and have higher minimum balance requirements. But this can vary based on the bank. For example, AmTrustDirect calls their account a money market account, but it has no check writing or minimum balance requirements. On the other hand, GMAC Bank calls their account a money market savings account and it allows limited check writing. It would probably make things clearer if all of these bank accounts were just called savings accounts. That would also help distinguish them from the money market funds.
Thanks to the reader who emailed me info on this article.