The reason for the scrub: a leaky valve at the fuel-tank end of a line that draws excess hydrogen gas out of the tank to be burned off at the end of a tall pipe some distance from the launch pad. You can read more about the particulars behind the delay here.
Have they found out why the valve gave them trouble and fixed it? No. They'll certainly try to. During a briefing this morning, launch director Mike Leinbach expressed confidence that engineers will identify the problem before launch.
But just to be sure Discovery stands a fighting chance of launching on time to achieve most of its objectives, mission managers have decided to replace the balky valve. The mission's major task is to install the station's final set of solar panels.
"If we can't find the smoking gun, it will be declared an unexplained anomoly, " Mr. Leinbach said.
Because it must rendezvous with the space station, Discovery is on a tight schedule. Because of the orbits involved, that means it has only a small window each month in which to launch.