Launch Director just announced that they have officially scrubbed this launch attempt due to a failed sensor in the liquid hydrogen tank. (A "booboo in the booster" for Bloom County fans.) This is one of four low-level cutoff sensors which would shut down the engine in an unusual case of low fuel levels to prevent the engine "running dry" and suffering a serious failure. But since this is an important safety system, the decision was made to scrub and detank the ET fuel in order to conduct an examination.
Update: They have not yet announced what the turnaround time will be for another attempt. The engineers consider this an odd anomolous failure and want to properly examine the sensor and all of its signal wiring. NASA says there will not be another attempt prior to Saturday; no firm schedule will be set until the fault troubleshooting is farther along.