It's also not a fallacy when an extensive history of similar encroachment exists. At some point, the burden of proof has to shift to the party who maintains that the government won't eventually abuse a given power.

IMHO there is no more certain indication that someone has been educated beyond their intelligence than when they sign onto an Internet forum and start braying about how the slippery-slope argument is a "logical fallacy." While technically correct, they betray an ignorance of how human beings -- a decidedly non-logical species -- actually behave.