UPDATE: The launch has been postponed for at least 48 hours. The delay has to do with technical difficulties.
Here are details on the difficulties from Launch Blog:
All technical issues that came up today have been resolved except for an APU Heater issue. If this is not solved by T-9 Minutes, Endeavour will have to stay on the ground.
The biggest concern is the weather. At KSC, teams will be watching winds that have to stay under a certain limit in case the Shuttle needs to land during an abort. The weather in Europe at the TAL abort sites is also being observed. The spaceflight meteorology group has issued a forecast that calls for showers within 20 nautical miles at all three sites: Moron and Zaragoza in Spain and Istres in France. For the AOA (Abort-Once-Around) scenario, three US landing sites are active: Edwards, White Sands and the Kennedy Space centre. Edwards and White Sands will be NO GO, so weather has to be just right at the KSC today.
EARLIER: The endeavour Space Shuttle’s 25th and final launch is scheduled for today at 15:47ET. This will be the second-to-last U.S. launch ever.
There have been a few hitches. Lightning struck near the launchpad during the night. Weather is good now, with a 70% chance of launch.
Recently there were reports of a liquid oxygen leak.