According to the Denver Tattler (Denverpost) The problem was related to a software update installed in 2004 that was not certified/tested after installation.
There have been no issues or none expected, just that the proper tests after installation were not complied. A 60-90 min check and certification and aircraft is back in service. Some of the grounded birds are already back in the air. No information about FAA's take on this at this time, UAL grounding is volintary. It might get nasty considering that FAA hammered AA on a previous issue. This apparently had been over looked since 2004.