Virgin Atlantic, which is being partly bough by Delta, and US Airways, which is merging with AA, are ending their frequent flier partnership effective on June 11, 2013. You can access Virgin Atlantic’s web page about this partnership here and US Airways’ here.
You can earn miles on each others program until June 11 and that is also the last day to redeem US Airways Dividend miles on Virgin Atlantic and vice versa.
Note that you can book award reservations beyond the June 11th date as far as the schedule is open and award availability exists. You cannot, however, change any awards on Virgin Atlantic issued by US Airways or awards on US issued by Virgin Atlantic after June 11, 2013, voluntarily. If there are flight changes/cancellations after the June 11 deadline, the airlines can work to accommodate you on an involuntarily change base.
Virgin Atlantic is getting a cash infusion from Delta that is also becoming its major shareholder. It was not unexpected that this partnership would come to an end. This was already announced back in January (somehow I managed to miss it).
This would have come to an end, for sure, once the merger with AA is finalized as British Airways, biter rival of Virgin Atlantic, has JVA with AA for the transatlantic flights.