Ansett used to fly A320 ETOPS from Perth to Christmas Island starting in 1993. According to their website they's not serving Christmas Island anymore.
Other airlines I know of that operate two-engine 100-150 seats aircrafts over long stretches of water are Air Pacific & Aloha Airlines.
Air Pacific is flying the 737-800 from Nadi to Vancouver; a distance of 10,000 km almost all over water. Click for large version
Photo © Andrew Hunt
Aloha have recently started flying the 737-700 from Honolulu to California.
So obviously there's a need for small ETOPS airplanes .