Update 2:12 p.m.: Regardless of the mission scrub, President Barack Obama decided to visit with the astronauts and Arizona Rep. Gabrielle Giffords. Biong Boing's Xeni Jardin tweets that Air Force One just landed at Kennedy Space Center.
Here's the president arriving on Air Force One, courtesy of @NASAKennedy:
It looks like the final launch of NASA's space shuttle Endeavor has been canceled for today. Space Flight Now is reporting a problem with one of the orbiter's auxiliary power units. NASA concurs via Twitter. The early word from the space agency is that there was an "issue with Auxiliary Power Unit 1 heaters." The mission will be delayed at least 72 hours. NASA has put the estimated launch time at 2:33 p.m. on Monday.
At least NASA's people appear to have a sense of humor about the delay. Jezebel founder Anna Holmes is at Cape Canaveral and tweets this quip from an unnamed NASA official: "We did show you a first. This is the first time we've sent out the Astrovan just to have it turn back." Here's a photo of the surprisingly old-looking van full of astronauts courtesy of Flickr user dpstyles:
It looks like astronaut Michael Finke bragged about his launch day breakfast a little prematurely.