This strange rhyme still used by children in English-speaking countries all over the world today used as a counting-out rhyme for games like tag actually have a very ancient magical and numerological origin. The very phrase "eenie meenie miney mo" harks back to a counting system older than the Romans, and possibly even older than the Celts. It is believed that the phrase was used by the Druids in pre-Roman Britain for precisely the same purpose: singling out an individual, but for a much more sinister and lethal end. Using exactly the same method, the Druids decided which of a group of candidates' number is up, and should be given as a human sacrifice to the gods.
Source: http://www.johnny.moped.btinternet.co.uk/origins/w/origins_whenyournumberisup.html