I've known something like this would be happening for over a year now. Back in December B&N filed for a number of trademarks, including one for Nookseller.
I cannot recall that they ever actually used that term, but I thought it suggested kinda what Overstock announced today. Update: A B&N insider left a comment and reported that the term is used internally.
And I've also been half expecting this ever since ever since June 2010, long before Pubit launched in October 2010. Barnes & Noble posted the contract details for Pubit in June, and I soon noticed the mention of "digital distribution partners". That term could mean any number of things, but I assumed that it meant that other sites would put their own brand on a Nook ebookstore. Admittedly, the Overstock deal isn't quite the same thing, but it's more than most expected.
But I do have to say that B&N puzzles me. Before this Overstock deal there was Books-A-Million. BAM is now the second largest bookstore chain in the US, and they sell all 3 Nook models. But they're the B&N Nook, not the BAM Nook, and I really have to wonder how B&N pulled that off. They have a brick-and-mortar bookstore carrying their product in stores. Surely this isn't a good idea for BAM, but this deal has been going on for a year now.