Dragon # 292 (The Get Wild/Dinosaur issue) has rules for it. I don't know if they are the "this is how we did it" rules or "this is how it works from now on" rules.
Anyway, Wolf is right. The weapon becomes the primary attack, and the natural attack is at -5 (and can only have .5 of the strength mod for damage, even if it normally is the full mod).
--Feral Spikey