After flying one million butt-in-seat miles on US (and credited to US) you'll receive annual Silver as long as you remain an active member of the program. See the bottom of http://www.usairways.com/en-US/dividendmiles/preferred/howtoqualify.html
So long as the miles survived the transition to the merged Dividend Miles program, they should, just as the old US "East" miles did.
I send an email to the preferred address about once a year, and they tell me my lifetime mileage total.
You can also call the preferred desk and they will look it up for you. Hopefully one day it will be under your account as it is with the other airlines