"Public-sector retirement costs also are high because many can retire at age 55 after 30 years of employment with pensions equal to 60% or more of final salary, which is often jacked up by lots of overtime in final working years. In some states, employees can "double dip" by retiring early and then resuming their previous jobs or taking other government positions. So they get salaries and pensions at the same time."
Here is a December 2009 article by CBS News on the topic as well. The article points to a USA Today news story indicating that the states of Utah, New Mexico, South Dakota, Florida and Arkansas are all looking to curb the practice through legislation.