Hmm interesting, not heard of this before. Its interesitng as I've heard its very good to wash down cactus with grapefruit juice as it cancels out some of the bitterness...

Also I've read quite a few reports of people that SWEAR that washing down mushrooms with lemon or blackcurrant juice make the resulting trip much stronger...I assumed this was down to the fruit acids converting the psilocybin to psilcybin so it hits you faster and stronger...well with grapefruit you would get these fruit acids and possibly this extra benefit...

Certainly interesting...I think experiments should be conducted! 8)