See my response to Mauling Monkey upthread on why the cold temperature of the Challenger launch actually wasn't the major issue it was made out to be.

Note also my comments there about other SRB designs that were known well before the Shuttle and the range of temperatures they could launch in. Those designs were used on many unmanned flights for years before the Shuttle was even designed. So in this respect, the unmanned test work had already been done. The Shuttle designers just refused to take advantage of all that knowledge for braindead political reasons.