NASA’s official page reads:
Based on early analysis of photographs taken during ascent, just a few items have been identified for further study. Some involve debris particles shed by the massive orange tank, which supplies liquid oxygen and liquid hydrogen to the orbiter's three main engines during the climb to orbit. The particles observed are small, and all occur after the time frame -- up to two minutes, 15 seconds into ascent -- that engineers consider to be the most aerodynamically sensitive time for the shuttle.Here’s hoping they come home safely.
About fifteen minutes into the flight, astronauts on board Discovery filmed a large piece of ice tumbling away from the orbiter. The ice, which forms on the exterior of the main engines, has been observed on previous flights and is not considered an issue.
"We don't see any concern for the orbiter," Hale reported, and added that in the coming days, the STS-121 crew will take a closer look at Discovery using the orbiter boom sensor system as well a slow back flip that will allow further photography of the orbiter from the International Space Station.