From the NASA website:
NASA is continuing extensive testing and troubleshooting into a problem with a low-fuel sensor in the Shuttle Discovery's External Tank.
Program managers have not set a new launch date, but say if the problem is fixed, an attempt could be made as early as four days later.
Firecrack: That isn't exactly easy to do. The Asteroid Belt is even further away than Mars. Any "rogue" asteroids closer than that are on wierd orbits, are moving very fast, and you want to keep them as far away as possible - we don't have the ability to slow them down or alter their orbits short of nuking the hell out of them and hoping, a la Deep Impact.
EDIT: UT, the shuttle had a complete systems replacement about 5 years ago. They got rid of all the old '80s equipment, and put in brand new digital stuff. Cost a
lot of money, and meant that the shuttle could stay in service rather than being junked sooner than it now will be. It was never meant to be in use for this long.

True, the shuttle has had two failures in just over a hundred flights, which isn't bad, but those failures were the result of design flaws, rather than just being accidents. With modern technology, a much better design could be created, which would be cheaper to launch, safer for the crew, and more flexible than the current vehicle. I'd much rather see NASA bods bite the bullet and commit to a new orbiter and launcher, rather than spunk even more money away on an old, flawed design.