Did one at Highland. Fantastic. Three hours of scanning with one resident, one other med student, and attending about 3x/week; tape review and journal club on Thursdays; attended resident conference on Wednesdays. Enjoyed myself immensely.
North Shore University Hospital has a very solid Ultrasound Elective. Great fellows and directors! Teaching and atmosphere is great. On my next rotation I did scans on my own and I'm very happy with the foundation they gave me.
Yale has a US elective. You can do 2 or 4 weeks. I haven't done it yet but will in Feb. I assume it would be open to outside students but don't know for sure. Ultrasound is pretty strong here, they just got a new machine and have 2 ultrasound fellows.