Just a few days ago I had made an award reservation SFO-BOS RT with the outbound flights on DL (the only perksaver available) and the inbound on NW. I was able to select seats for the NW flights and then went to delta.com to change the seats for the DL flights. Then the Mountains of Miles promotion came up, so I canceled that reservation, redeposited the miles and booked the same exact ticket again in order to get the promo points. ... Low and behold! Now I was able to select seats for the DL flights as well at nwa.com. Very cool! ^

(Of course, flying DL vs NW still sucks, but still...)