I have spread this rumor myself on several occasions, but recently learned that it's a myth. Mythbusters themselves tested it out, and found that when extremely threatened Daddy Long-Legs can bite humans, but their venom isn't very harmful at all. Scientists have found that their venom isn't too poisonous to other insects as well.
Good thing they have a back-up plan: Daddy Long-Legs can make their bodies vibrate extremely quickly, making themselves an almost-invisible blur to enemies. Not a bad idea. :)