The answers to Adam's original question have gone all over the place, mostly due to either incorrect answers or unfathomable ones. Hopefully the following information will help set the record straight:
1. Aluminium ('Alu') is LIGHTER than Titanium ('Ti').
2. Ti is STRONGER than Alu.
3. Because Ti is the stronger element, certain components made from Ti can be manufactured using less material than if they had been made from Alu.
4. Therefore, even though the base material used is heavier, because less of it is used, the part will be lighter.
5. Point #4 above is NOT relevant to bolts, because you cannot manufacture a bolt using less material -- after all, the bolt must fill a pre-determined space. (The exception is if the bolt is hollow, but the size of bolts used on bikes and their required strength means hollow bolts are not relevant.)
6. Therefore, an Alu bolt will always be LIGHTER than a Ti bolt of the same specs.
7. It also means that the Ti bolt will always be STRONGER than the Alu bolt.
BTW, I am fully aware that there may be some very minor inaccuracies in the above explanation, but they are irrelevant to the gist of the discussion, so I allowed them...
Regarding which bolts should be Ti and which should be Alu, this depends on how much of a trade-off one wants between saving weight, and safety. An extreme weight-weenie may be willing to change all his bolts to Alu, whereas a safety-conscious one will prefer to use only Ti or steel. For accepted practice as to which bolts can be Alu and which should be Ti, check out Nino's bolt article at http://weightweenies.starbike.com/articles.php?ID=16
I hope this helps...