And of course, if you change the Referer header to http://www.google.com/, you get a different page with the solution at the bottom.

I find myself wondering though - why don't they get punished more for feeding different content to the GoogleBot vs what the normal viewer sees? Isn't it basically cloaking - even though clicking on the link in Google search results will still serve up the solution, a normal, organic link wouldn't?

http://googlewebmastercentral.blogspot.com/2008/06/how-googl...

"Cloaking: Serving different content to users than to Googlebot. This is a violation of our webmaster guidelines. If the file that Googlebot sees is not identical to the file that a typical user sees, then you're in a high-risk category. A program such as md5sum or diff can compute a hash to verify that two different files are identical."