[TECK]

Stefan, from my experience with search engines (and I did searched a lot), indexes will drop your site for the following reasons:
1. no unique meta tags for each page (most important)
2. popups or any other external links, that direct your crawler outside the site
3. deprecated html code

Now, others will argue with me and tell me: "heh my archive is listed..."
Let me know for how long. I guarantee you that by the end of the month your links are dropped.
The best way to test your links is to go to Google and search for:
site:yoursite.com archive

So, I strongly recommend you Stefan to update your code, related to those issues I mentioned above, orelse the archive pages will NOT be indexed.
vBulletin.org decided to install my archive because of "no. 1 reason", listed above.

I know that "some" people (same who rated 1 my archive) will consider this as "spam" or "advertising" my own code... believe what you want, I simply try to help.
Feel free to read more here to see that what I say is true:
http://www.searchengineforums.com/