Hmm, three ways to approach this issue with ranged weapons and enchantments.
1) The bow is being treated as a sort of 'double-weapon', and it's "melee head" isn't enchanted yada. However, that would still result in an improvished weapon that should be masterwork since I don't believe you can make only half of a double-weapon masterwork. ( No Bane bonus )
2) Using a weapon as an improvished weapon is simply another 'function' of the weapon (Such as Trip Attacks). And since I'm pretty sure you apply your enhancement bonus to your trip attack attempt, the enhancement bonuses and other magic effects on the bow should apply. ( Yes Bane Bonus )
3) Text line from 3.5 SRD Bane.
Bows, crossbows, and slings so crafted bestow the bane quality upon their ammunition.
This may be interpreted as meaning that bows, crossbows, and slings with this enchantment ONLY bestow it upon their ammunition. ( No Bane Bonus )
Interesting sidenote, in 3.0, it would appear the only way for a Dragon-bane bow to be effective, would be when you're using an improvished weapon (it isn't in the ranged weapon specials, nor does it convey its bonus to the ammunition).
Honestly, I'd lean toward #3 for justification, it's the sleakest, and kinda makes the most sense to me. It also keeps from implying that bows/etc are somehow "enchanted twice" to both enchant their ammo and the weapon itself.