[Sir Michael]

The bow itself is not a weapon; it is just the launcher for the actual weapons - the arrows. Short of having a bow which generates its own arrows, here is how it works:

A magic bow may give bonuses to hit and/or damage, but normal arrows themselves are not considered magical, and so will not damage creatures only hit by magic weapons.

When magic arrows are used with a magic or non-magic bow, the bonuses (if applicable) stack, and any damage (e.g. fire arrows) also applies. If using +1 arrows, monsters susceptible to +1 (but not +2) weapons may be hit, and so on.

Note that I think that some arrows without specific bonuses to hit, like fire or cold or acid arrows, may also not be able to hit monsters which are only damaged by magic weapons, but I'm not sure.

Magic arrows are cheap and readily available, so why not keep a supply on hand to use against those monsters, whether or not you use a magic bow?