You can't use B&N's ebooks on Sony.
B&N's ePub is a confusing issue. You can read this post on Teleread about it. The comment by Jim Lester explains . . .
http://www.teleread.org/2009/10/20/b...omment-1147268
Basically, B&N's ePub will have its own DRM that isn't currently compatible with Sony's firmware (everyone else's too, even ADE 1.7). It'll be up to Sony to decide if they want to use RMSDK 9.1--the new version of ePub DRM.
My guess is that Sony will allow this since they already allow users to purchase ebooks from other retailers.
Also, it doesn't sound like this new version of ADE and firmware update will happen until early 2010.