Simon & Schuster's website apparently gives you a choice of Adobe (no idea if this is PDF or ePub by default, though the one listing I tried did have just ePub), eReader, or Microsoft Reader formats, according to
their help page. You may be better off getting any of their books you want through one of the regular major bookstores, or testing on their giveaway freebies first to see what you end up with.
B&N actually only have two formats under the same DRM scheme, and they've been trying to switch exclusively over to just the one.
But their grand plans for going all ePub and killing off the eReader format hit a snag when they failed to upgrade their Mac reader software so as not to crash with their shiny new default DRM-ed ePubs (non-DRM ones work just fine).
Also, they still have to support Blackberry and they haven't gotten their app for that to do ePub yet, either, so PDB lives on to haunt them, undying like a zombie!
However, Windows users will only ever see ePubs, under the current regime. They really ought to give one a choice of file format at the website.
I'm sure there are still some non-Mac users with eReader-compatible devices who'd love to be able to get directly usable book downloads from B&N. And a sale is a sale, no matter what it's going to be read upon.