Endeavour and her STS-134 mission to the ISS has been pushed back 10 days to April 29, 2011. The change was necessary due to a conflict with a Russian Progress supply vehicle scheduled to launch April 27 and arrive at the station April 29.
You can read more at http://www.nasa.gov/mission_pages/shuttle/main/index.html
The STS 134 Crew Takes a Break at the Pad
Sky Guy in VA