My (unfortunately significant) experience with water pump housing bolts snapping (and _specifically_ water pump housing bolts) has led me to believe that you're likely to be able to spin it out with vise grips once you can grab it.
It seems like corrosion builds up between the bolt and the housing, which grabs the bolt shaft tight - snap! However, every time this has happened to me (6 or 7, at least), once I get the other bolts off and slide the housing off, the remaining bolt is A) sticking out pretty far - the head has always snapped off, not leaving it flush with the block/head and B) they come _right_ out. Weird.
Now, if yours snapped off flush, uggh. If it is going to a machine shop, yes, have them do it:).
Iain