B&N ebooks are in the eReader format which they now own after buying FictionWise. FictionWise has talked about switching eReader to ePub, but this may be a year away and even if they do so the DRM (encryption) will likely still be incompatible with the Sony Readers. B&N has a press conference on the 20th that may clarify their future plans.
It is possible to strip the DRM from eReader ebooks, see
eReader Decoder Tool. This requires Python and the command line, but it isn't in the end all that hard to use. Once you have a DRM-free ebook, Calibre can convert it for you to ePub or LRF for the Sony. However, even with the low prices that B&N has for some ebooks, you may be better off buying ePub ebooks from other stores.