The Nook is the first mobile ADE device to allow user-selection between multiple system fonts. However, Liza points out that it does not work very well. I think B&N needs to redefine both serif and sans-serif to whichever font is selected. If there is an embedded font, this obviously still won't have any effect.
It also a bit strange that B&N isn't offering the
Droid font family for ePubs. Liza points out that language support is better in the library than in ADE (the library is probably using Droid, including DroidSansFallback).