ive thought about this topic alot recently because my body is very tolerant to caffeine, i would have to drink about 3 or four cups of coffee to feel the least bit affected by the caffeine. I found many problems with caffeine, yes you could 'overdose' on caffeine. Here is a link to a known occurance
http://www.everybody.co.nz/research/nut_caffsens.htm
the LD_50 of caffeine, meaning the letahal dosage for 50% of teh population seems to be about 150mg/kg.
BUT, research has found fewer suicides among cffe drinkers than non coffe drinkers, caffeine also increases the level of circulating fatty acids. This has been shown to increase the oxidation of these fuels, hence enhancing fat oxidation. It's particularly effective in those who are not habitual users.