I visited two of the former US Clubs in PHX this weekend (first to hang with a friend at the lounge near his gate and then to wait for my own flight on a different concourse after exploring for a bit). They seem to still be using a US computer system, and each lounge had a supervisor/trainer type person on hand to help the attendants check people in.
In both lounges they commented about how their computers can't yet swipe the Prestige cards, so they had to go through several screens on their computers and then manually type in my card information, but they figured it out (with the supervisor/trainer's help). Neither lounge asked to see a boarding pass so I didn't get to test whether they'd give me a hard time about my US bp (although I was booked through AA on the AA codeshare).
I'll have another opportunity to test the US boarding pass issue in a few weeks at EWR.