Can one assume the NV GECK will be backwards compatible with FO3 content?
It will not be backwards compatible. In addition to numerous changes in how it works, there is no doubt scripting changes and kernel changes and engine changes that would make loading a Fallout 3 Bsa, if it is even allowed, impossible.
Also, and this is very important, resources from Fallout 3 can not be used in mods for New Vegas. Period.
Only files that are in the included BSAs, any new content released by Obsidian (not saying there will be, but if there is any) can be used. Unless Bethesda states a policy change, this is the way it has been with every game they make an editor for, no content from other games can be used in their game. This does not mean someone could not model something and create it from scratch to look like something else from another game, like new weapons, but that is original content by the modder, and that's OK. I think there are some mods that bring in some Fallout 2 weapons into the game with new meshes and textures, just using the existing animations and models. Since they are created from scratch, those mods are allowed even though they are an homage to another game.