If you really want to allow Druids to use bows (which I wouldn't recommend), don't bother taking something away. Just make the Druid use a Feat to take a proficiency with it.
IMO, just dropping the restrictions altogether would be fair. They'll still have to spend a feat or multiclass (sucks for a caster) to become proficient.
I've never really understood why druids can't use other weapons. The justification in the PHB just doesn't seem quite satisfying when a cleric can burn a feat and swing a spiked chain around (or a wizard could use a friggin' greatsword - and no, I'm gonna get into what a "friggin' greatsword" does). It sure doesn't seem like a game balance issue when the other spellcasting classes don't have that limit on them.
Didn't the 3E playtesters say that the druid weapon restriction weren't there for balance, but for nostalgia/flavor?
Would on of the 3E playtester care to comment? I might be wrong, my memory seems to be slipping in my old age
IMC, I dropped the weapon restrictions. They still have to blow a feat or multiclass to use bows, but they don't lose their druidic powers for the day for doing so. Hasn't upset game balance at all, at least not yet. Coupled with some of the non-PHB druid spells it might, so I'm pretty particular as to which ones they can use, though.