At a whopping 27 miles, is this segment (a tag-on) the shortest flight in the US? It seems like the shortest flight in the UA system.. but I might be wrong.
Based on the posts in a thread in 2005, noone named a current shortest flight..
JST-AOO may be the shortest in the US though I understand there once were scheduled helicopter flights from LGA JFK and downtown NY to JFK. Those probably would not count if you are looking for flights today.
Era Aviation (more specifically, Hageland Aviation) flies BET-KWT (Bethel, AK to Kwethluk, AK) non-stop on Cessnas at a distance of only 13 statute miles. There may be other routes even shorter, particularly hopping between Alaskan villages, but this was the one that came to mind.
They are indeed scheduled flights (and bookable to the general public at Era's website). I should clarify, though, that the segment is only flown non-stop BET -> KWT. The full routing is flown 3 times daily as a circle route of BET-KWT-KKI-AKI-BET (Bethel-Kwethluk-Akiachak-Akiak-Bethel).
Now that I think about it though, the shortest non-stop segment on that routing is actually KKI-AKI at 7 statute miles.