[Andreas]

http://www.google.de/intl/us/webmasters/guidelines.html

Make pages for users, not for search engines. Don't deceive your users or present different content to search engines than you display to users, which is commonly referred to as "cloaking."

(I am pretty sure other search engines have similar terms)

If a search engine indexes a Thread that is only accesable for Registered Users (or even worse only for paying users), and I access your site as a guest, I do get different content (a no permission error).
As you are intentionally allow search engines to spider content that is not publically accessable (to attract more users than there would be if you only had public content indexed), this is to be considered cloaking.

The search engine friendly archive is different in 2 ways:
a) I do get the same content through the archive as I do get through the forum
b) If I, as a Guest, access the archive I do get the exact same output as does a search engine spider.

Of course there are thousands of sites out there that are cloaking and still are indexed ... how many pages does Google have indexed?
Just because they are not discovered and remove yet, does not mean that they never will be.

If you come across sites that are cloaking, report them - this increases the chance of getting them removed.