Seems they have all been replaced with the ex-NWA 320/319 aircraft, more 738s, and Skywest CR9s. Before the merger it seemed DL had all 16 MD-90s based in SLC, but obviously they have since acquired more used frames from other carriers and seem to be using them in ATL, DTW, & MSP. A commuting captain told me several years back that DL liked the aircraft and they performed well in high altitude conditions.
Josh
Josh