Itinerary was as follows:
10/3/09
DL 1548: LAX-CVG
DL/Comair 6930: CVG-PVD

10/5/09
NW/Masaba 3498: PVD-MSP
NW 652: MSP-LAX

Checked in online and flew my outbound DL segment, LAX-CVG and CVG-PVD with no problems (well, I was as comfortable as one could get on a 100% packed red-eye from LAX to CVG on a 737. Blah!).

For my return on NW metal, PVD-MSP and MSP-LAX, when I attempted to check in on DL, it redirected me to the NWA site. There it displayed my inbound itinerary, a unique NW PNR number and had a "check in" button that I clicked. It then directed me BACK TO THE DL WEBSITE where I was provided a list of my inbound itinerary which included CVG-PVD, a segment I already flew two days earlier. Even my miles from that segment were already posted on the DL website!

When I clicked "check in" it gave me a message that I have to check in with Comair (huh? How does one do that?), despite having already checked in and flew my Comair segment using the very DL website that's now claiming I have not checked in for that flight, let alone completed it.

I then went to the NWA site and checked in using the NW PNR number and did that successfully.