You still need a rooted version to access the Android app store (and probably always will since they won't want people buying this to install other eReader apps). It's a shame they need a whole new app store mainly to prevent access to a handful of apps.
The nook lacks hardware features (a camera, at least) that google requires for a device to qualify for google branding and google apps including android market.
The rules are based on the OS version. There are some differences for 3.0, but in this case 2.1/2.2 doesn't officially support tablets at all so there weren't any exceptions. Froyo did require a camera, and the four android buttons - I don't think the nook has those either.
