The Player's Handbook describes the bolas as
two or three heavy wooden spheres connected by lengths of cord. Because the bolas can wrap around an enemy’s leg or other limb, you can use this weapon to make a ranged trip attack against an opponent. You can’t be tripped during your own trip attempt when using a set of bolas. (115)
But the weapon is also listed as dealing 1d3 points of nonlethal bludgeoning damage on Table 7–5: Weapons (116-7).
Is the ability of a bolas-thrower to make a trip attack at range a separate and distinct feature of the bolas that must be picked when a set of bolas is thrown or, instead, a feature the bolas-thrower can opt to use in addition each time a set of bolas is thrown? That is, when a creature proficient with the bolas hurls the bolas at a foe, what happens? Below are two possible scenarios.
- The creature picks either to make a normal attack roll with success meaning that the weapon deals 1d3 points of nonlethal bludgeoning damage or to make, like an otherwise normal trip attempt, a touch attack roll yet at range then, if successful, an opposed check (the attack's Strength check versus the better of the foe's Strength check or Dexterity check) with the attacker's victory tripping the foe.
- The creature makes a normal attack roll with success meaning that the weapon deals 1d3 points of nonlethal bludgeoning damage and then makes an opposed check (the attack's Strength check versus the better of the foe's Strength check or Dexterity check) with the attacker's victory tripping the foe.
Answers should feel free to develop additional scenarios beyond these suggestions. I'd prefer hewing as close as possible to the printed rules, but, absent any printed rules (or if they're nonsense or utterly unclear), house rules are fine.