It now appears that the earliest that the Space Shuttle Discovery can be launched is July 26. They still can't figure out what's wrong with the sensor. MSNBC.com is reporting that:

Discovery's sensors have been acting up for months: The system flunked one tanking test in April, then passed another one in May. Then Discovery's tank was swapped in June for a newer heater-equipped tank, and the launch team had thought the problem was solved. During last week's countdown, however, one sensor started exhibiting bad readings on an intermittent basis.

Now that the tank is empty, the sensor seems to be working as expected — complicating NASA's troubleshooting efforts.

"It's difficult to find a glitch that won't stay glitched," Parsons said.

If the troubleshooters are unable to find the source of the problem, NASA managers could reconsider their rules to allow for a launch even if one of the four low-level sensors isn't working, Hale acknowledged. "We're thinking about it," he said. But for the time being, the mission management team was putting aside that scenario, pending the outcome of the upcoming tests

Isn't that the way it is with our cars? Or computers? Something goes wrong, but as soon as we call the repair person, the problem can't be found.