Marvel released a press release earlier showing off the cover to Secret Avengers #12.1.
Originally, it was going to be the variant cover to Secret Avengers #6.
Guess, that means - maybe - Nova was supposed to keep avenging?
Then he seemingly died at the end of The Thanos Imperative and hasn't been seen since.
So the replaced him on the cover for Black Widow.
What I fear has become a reality.
Side by side, check them out for yourself. Secret Avengers #6 Variant - Secret Avengers #12.1.
Updated 2/13/11 with Tom Brevoort's answer via Form Spring:
CBN: We see SA12.1 cover was the original SA #6 variant with Nova on the cover. Was Nova supposed to be a regular member in SA?
Tom Brevoort: It was actually an inventory cover that we ran in the Previews catalogue for #6 when the main cover for #6 wasn't completed in time. And it was done at the time when Nova was int eh book, hence the reason he was on the earlier versin of thei piece--since we're using it now, after Nova is gone, we had Deo replace his figure.