Alcohol was legal then illegal, and was re-legalized
The illegalilzation of alcohol during the Prohibition, and its subsequent re-legalization when Prohibition ended, suggests that such reversals are possible and that therefore the legalisation of drugs need not be irrevocable.
From a comment by Mike Barskey (12/1/2008 2:11:00 PM GMT):
"The Prohibitiion in the US illegalized alcohol, which is essentially where the US government is today regarding drugs. When the Prohibition ended, alcohol was re-legalized (thanks a lot to jury nullification). Why would the reversal of the current drug prohibitions be "probably irrevocable?"