There are many common chemicals we put in our foods that have a higher LD50 then THC (if you don't know what an LD50 is google or wiki should fix ya up quick).

You are more likely to die of a salt or caffeine overdose then thc. A recent study in the UofSask. put ultra high dosages of pure synthetic THC into rats (a dosage that would be impossible to get through smoking) and after prolonged treatment the rats had BETTER memories then control rats.

While this was not the expected result they did hypothesize that the THC stimulated certain parts of the brain by increasing blood flow to those areas, areas which are attributed to long term memory. Temporary short term memory loss for permanent long term memory gain? If only I can get my hands on some pure synthetic thc... mmm.....

THC is about as harmless as it gets.