Greg Bear underwent emergency open heart surgery on September 23.
A public announcement was made by organizers of the San Diego Comic Fest. Bear was to have been its Science-Fiction Guest of Honor in October:
Greg is one of the finest people in science fiction and a great author and we look forward to seeing him as a guest at a future Fest.
He reportedly is doing well post-surgery.
Update 09/26/2014: Bear’s wife, Astrid, posted on Facebook that he was diagnosed with an aortic artery dissection. The aorta was surgically repaired and a mechanical aortic valve installed. By the next morning he was off the breathing tube and capable of sitting up in a chair for half an hour.