Domestically, all nonstops between JFK and LAX/SFO, HNL and MSP/DTW/ATL, and I believe select flights on other routes such as DCA-LAX, JFK-LAS and JFK-SEA.
The flat bed is only ONE element of what Delta markets as DeltaOne. Others include SkyClub access (which you don't get on an ATL-SLC widebody), enhanced catering, bedding, amenity kits, and restrictions on the timing of medallion upgrades. It's DeltaOne if the website says it's DeltaOne when you book.