This afternoon SpaceX sent out a press release announcing May 19 as their next launch attempt date:
SpaceX and NASA are nearing completion of the software assurance process, and SpaceX is submitting a request to the Cape Canaveral Air Force Station for a May 19th launch target with a backup on May 22nd.
Thus far, no issues have been uncovered during this process, but with a mission of this complexity we want to be extremely diligent.
We will send out updated information on launch activities in the coming days.
A Johnson Space Center tweet stated the launch time will be 4:55 AM Eastern Daylight Time.
As noted by SpaceflightNow.com, the two-week delay allows time for Roscosmos to dock a Soyuz capsule at the International Space Station with three new crew members.
UPDATE May 5, 2012 — NASA issued this press release in response to the SpaceX announcement:
"After additional reviews and discussions between the SpaceX and NASA teams, we are in a position to proceed toward this important launch. The teamwork provided by these teams is phenomenal. There are a few remaining open items but we are ready to support SpaceX for its new launch date of May 19."