Is not the design as well as the specific textures and meshes also copyrightable? As far as I know, it is. Or I could just draw Mickey Mouse with a different type of pen and Disney couldn't do a thing about me.
Even copying likenesses and designs is, as far as I'm aware, badness. That is, unless the design is pre-dating Obsidian's creation of it. So copying NV's Plasma Caster/P94 is naughty and bad and illegal, but copying the 9mm pistol is not, as it's a clone of a real world weapon, which neither Obsidian nor Bethesda own the IP for.
That's the safest thing. To the extent that FONV (haven't played it yet myself) uses new real-world weapons, that's pretty safe to port to FO3, just not using any of the FONV art or design. Unique weapons or items would definitely be much more legally dangerous. It's still nothing clear-cut, because copyrights most clearly apply to those who take an item an duplicate it with no creativity added, and then sell it. In the context of a mod, there IS creativity added, since it's being integrated to a different game with its own unique level list and such, and it's not being sold for profit.
In general, I'd like to think that anything that's respecting the developers' work on both games, and the developers' rights to profits FROM both games, will in turn BE respected. But in general, better safe than sorry for us all.