I just got back from a trip that was booked SAN-ORD-GSO-CLT-ATL-CLT-GSO-IAD-SAN, with some of the segments on UA and some on US (the CLT-ATL-CLT-GSO).

The ticket was on US ticket stock, but all of the UA flights plus ONE of the US flights showed up in "my itineraries". Conversely, when I contacted US, only ONE of the UA flights showed up in that itinerary.

The only guarantee seems to be that if it's on UA metal and your MP# is in the record, it's going to show. Otherwise, I think it's somewhat of a crap-shoot.