It’s easy to forget that US Airways and Virgin Atlantic are partners, since you can’t actually redeem US Airways miles for anything other than economy. Per the award chart (.pdf),
Award redemption on Virgin Atlantic is only available in economy class.
Even that option will be going way, and indeed you will no longer be able to credit Virgin Atlantic flights to US Airways Dividend Miles effective June 12.
After June 11, 2013, you will no longer be able to accrue or redeem miles on Virgin Atlantic. For travel before June 12, 2013, please include your Dividend Miles number when you book or check in. Please submit all requests for retroactive miles before December 11, 2013.
That’s a shame because the US Airways program is much better than the Virgin Atlantic one, so this was a great option for crediting paid tickets on Virgin.
And of course the reverse is true, you will no longer be able to credit US Airways flights to a Virgin account or use Virgin miles for flights on US Airways.
I have no special knowledge about the reason for unwinding this relationship, although of course Delta has acquired a 49% stake in Virgin and so presumably the explanation lies there.
If US Airways merges with American it would lose the Virgin Atlantic relationship in any case, first because the surviving program will be American AAdvantage which is not a partner of Virgin and also because American is a oneworld partner of British Airways. That could bring US Airways full circle, as in the early 1990s British Airways had made an investment in US Airways forming one of the first transatlantic alliances, which ended when British Airways partnered with American instead.