From 3.0e main FAQ
If a barbarian became undead (by gaining the vampire template, for example), how would you determine how long his rage lasts? He no longer has a Constitution score. Could an undead barbarian theoretically rage until all opponents were defeated or someone takes him down? Or does his rage simply last 3 rounds (base 3 + 0 for no Constitution score)? Or do you assume that the undead barbarian has a Constitution score of 4 (0 + 4 bonus from rage) and so cannot rage at all, since the modifier for an ability score of 4 is −3? Does this Constitution “boost” have any other effects on the undead barbarian? Does he actually lose hit points?
An undead creature uses its Charisma modifier wherever its Constitution modifier would normally apply (except Fortitude saving throws, for which the creature must use its +0 Constitut ion modifier if it needs to make a Fortitude save at all). Thus, the example barbarian rages for a number of rounds equal to 3 + his Charisma modifier (but always for at least 1 round). An undead creature has no Constitution score at all and cannot gain one by receiving a bonus. Therefore, the +4 Constitution bonus from rage has no effect on this barbarian.