Technically speaking the code to run bows and arrows may still be in the engine, or the arrows fired may simply be a regular object that uses havok physics to simulate flight-decay? I don't honestly know. From a legal standpoint, if they left the code in the engine and if we can access it via the GECK, then it's legal to use. It's the content that must be New Vegas-specific or home-spun by a modder, nothing from Oblivion or Fo3 (that didn't transfer to FNV) can be used in New Vegas mods. What we don't have are the Animations for players and NPCs to shoot bows - those are Oblivion-specific and can't be used. This makes Crossbows a logical mod-target as they can be wielded like some guns, and modders can use existing animations to make them work. Given that we already have one crossbow mod made, I have to assume it will also be possible in New Vegas using animations from NV for guns that are held and fired in a similar-way. It won't likely look quite-right unless someone makes an animation for them, but they will still work.
It's a lot of work to be sure...