If I recall correctly, it used to be the case that they only showed the answer to Googlebot and if you wanted it, you actually did have to pay (or perhaps forge your useragent)
I think Google added to their policy that if your site returned different content specifically to the Googlebot, it would have an impact on whether that site would appear in search results.
Experts-exchange got around that by making the answer appear at the bottom of the page and required a lot of scrolling.
It used to be the case that if you weren't Googlebot, you'd see some blurry scrambled text where the answer ought to be. The blurriness was created by superimposing a partially transparent image over the text, and the scrambled text was really the rot13'd answer, so it was possible to get the answer by grabbing the relevant chunk of html source and running it through a rot13 tool.
