Clarification/Follow-up by Jeramie on 11/15/06 9:54 pm:
Only on the first 4 shuttle flights, which only had a 2-man crew. That's why the shuttles have those two "sun roofs" on the top, as ejection ports. However, once the crews began to be larger than two people, the seats were removed as infeasible for the entire crew.

Clarification/Follow-up by denberg on 11/22/06 2:19 pm:
Thanks Jeramie, that's a perfect answer. Thanks.