|Bumping this one up because it's currently more useful than the other spoilers-related thread doing the rounds.|
So, we've got the option presened here - making all signs of text invisible - or the one in this thread - sticking a great ugly lump of black space in the middle of a thread. If it came down to a choice between the two, I'd take the first over the second any day of the week.
If it's got to be done manually, the only solution I can see to the issue of knowing where the text actually is would be to do $$something like this$$ - indicate spoiler text with a recognised identifier.
Maybe non-moderators could use the black masking style and then moderators alter it to the invisible style by editing posts? That's going to be a shitload of work if people start spoilerising all over the place, of course.
Oo. Nother suggestion. How does using the alternating colours work? Having spoilers in the dark forum colour when in light posts, and vice versa.
Test: Dark on light.