Reviewing the board, there seem to be two candidates for shortest domestic flight currently served by a mainline carrier in a jet aircraft:

ITH-ELM on US (F100, flts. 305, 526, 1175)
WRG-PSG on AS (737, 1 RT daily)

Both check in at 31 miles according to http://www.chicago.com/airliners/gc.html Haven't seen any int'l flights shorter than this but I didn't look that hard.

I once took the USAir flight ELM-ITH about 8 years ago; it was on a DC-9.