Barnes and Noble WAS offering epubs but appear to have (temporarily?) switched back completely to pdb due to the problem that certain of their mobile phone readers didn't actually support epub files. People who purchased books and got epub and redownloaded them are now getting pdb files.
http://bookclubs.barnesandnoble.com/...pdb/m-p/440125
Nobody knows if you'll be eventually able to redownload the same content in epub or not in the future, or if the format will only apply for new purchases once they switch back over.
Personally, even though I have a Sony I was finding it EXTREMELY useful to download book samples (in Epub format) from Barnes and Noble, which were not DRM'd, in order to preview how the epub would work on my device. Now all the samples are pdb as well, so I'm just waiting it out until they switch back. Keep an eye out for an update to their mobile reader software; I'd expect that to precede any switch back to epub.