Y2K7 fears
From CNN:
NASA wants Discovery back from its 12-day mission by New Year's Eve because shuttle computers are not designed to make the change from the 365th day of the old year to the first day of the new year while in flight. Engineers have developed a fix but are reluctant to try it because it hasn't been thoroughly tested.
I can absolutely appreciate the difficulties of managing time with computer software (we've been having lots of problems lately at work handling time changes with software) but this seems like something that should have shot to the top of the to-do list as soon as this shuttle flight was scheduled. A shorter shuttle flight is better than no flight though.