The U.S. Space Shuttles are fragile beasts, with (so far) a catastrophic failure rate of about 1-in-60. For that reason, NASA takes even small amounts of damage to the shuttles very seriously.
When a severe storm pounded Florida a couple of days ago, Atlantis was already sitting on the launch pad, and consequently the foam protecting its external fuel tank was damaged by severe hail. Since degraded foam doomed Columbia a few years ago, NASA has taken damage to the foam protectant very seriously.
As a result, NASA has decided to roll the shuttle back into the Vehicle Assembly Building to repair the damage, which will result in a roughly six week delay in launching the shuttle (they cannot launch to the ISS while the crews are being changed out due to traffic congestion). So the new expected launch date for Atlantis will be in late April.
By the way, I'm not an expert on launch pad processes, but it seems to me that rolling the shuttle out to the pad a month before launch is just asking for this kind of trouble. This isn't the first time NASA has had to delay a launch because of something like this. I know they get to have drills and practice the launch a couple of times by having the launcher on the pad, but do they really need a month to do all of that, or could they be server by having it out 7-10 days before launch?