Well there was a report in our papers this morning that it could have been caused by sprite lightning. Apparently sprite lightning is a very rare form of lightning that only happens way up high. Some guy took several pictures, that haven't been released yet, of the Shuttle just before the incident, and there was a strange right angle of what looked like lightning. Apparently NASA is interested in this.
Edit: Oops, just seen this thread about cosmic bolts.
http://www.techimo.com/forum/t51783.html