I recall that plan and thought it was a good one (and still do). However, it has no effect on search engines. marto's suggestion means that the content would not be indexed in the context of the thread which would be even better. Reducing the font size also has no effect on readers with character-cell browsers (lynx/elinks/etc) or screen readers (so there's a WAI concern).
Perhaps we could do both? Below one threshold (-3? -4?) start progressively reducing the font size and then below a second threshold (-8? -10?) hide the content entirely.