OK, to contradict what I said previously...
Saturday I did a same day run, SFO-MSP-ATL-MSP-SFO all on NW metal. I was able to check in at the 24 hour mark for the SFO-MSP-ATL segments from the my reservations page. At the 24 hour mark for the ATL-MSP-SFO segments, I couldn't check in from the my reservations page, but was able to check in from the home page by entering the conf # and departure city. Oh, and I had the pleasure of sitting next to a fellow FT'r on the MSP-SFO leg!. Anyway, was able to check in on-line for 4 segments on the same day this time at least.