Creatures with natural attacks and attacks made with weapons can use both as part of a full attack action (although often a creature must forgo one natural attack for each weapon clutched in that limb, be it a claw, tentacle, or slam). Such creatures attack with their weapons normally but treat all of their available natural attacks as secondary attacks during that attack, regardless of the attack's original type.
Secondary attacks are made using the creature's base attack bonus –5 and add only 1/2 the creature's Strength bonus on damage rolls.
Some creatures treat one or more of their attacks differently, such as dragons, which always receive 1-1/2 times their Strength bonus on damage rolls with their bite attack. These exceptions are noted in the creature's description.
Reading that together, I believe you would treat the bite just like any other natural attack. If you attack with a weapon and the bite, the bite is a secondary attack for that round (so 1/2 STR bonus only and -5 to hit). That's how the stat entries for creatures that have similar set-ups work (Babau, for instance). Could be wrong, though.