There's nothing in a "slippery slope argument" that makes it technically invalid. Improper use may make it fallacious, but there are also real slippery slopes and ways to deal with them (e.g. Schelling fences).
As for being a slippery slope argument, people might have dismissed this story in .nl in the 1920s for the same reason, yet it happened.
This argument refuses to take sides. I'm not "for the terrorists" with this argument (nor am I against them), I'm not "against the government" (nor am I a clear supporter), I might be slightly paranoid (which makes for a better discussion than being a paranoid who looks out for black helicopters).
With this argument I'm all for being careful, and it even leaves a door open for discussion how surveillance and data mining could be built in a way that such catastrophic effects won't happen - who would argue against that?
But suddenly, it's about the practical issues of surveillance and data mining.
