I just checked it on my trip out of SFO, but no luck. I couldn't complete check-in on line anyway, though, since I "can't show proof of onward travel" (despite that onward travel also being booked on UA with my MP number and showing up in My Trips). Would have been nice! Hopefully it works for the way back!

ETA: I actually just checked the travel ready center for the return flight, and it is working there. Sweet!