Bloomberg reports that as the yields of Treasury securities continue to fall, and investors flee for the perceived safety of money funds, money market mutual funds investing mostly in U.S. Treasury securities have started to turn away new investors to protect fund yields. Shutting out new investors protects existing investors in the fund because the fund does not have to purchase new treasuries with lower yields than those already in the fund's portfolio. Higher fund yields also generate higher management fees.
This December 4, 2008 Bloomberg.com article by Miles Weiss and Christopher Condon provides some details on the larger funds that have closed to new investors, and some of the prevailing conditions driving those decisions: http://www.bloomberg.com/apps/news?pid=email_en&refer=home&sid=aq_FV92tPkRI