I've been seeing many questions -- and especially many concerns --
about our supplemental index.

This FAQ gives a nice albeit brief overview:
http://www.google.com/support/webmasters/bin/answer.py?answer=34473

We may want to update that FAQ with some additional info later, but in
the meantime, I thought I'd clear the air a bit:

1) Penalty?
When your site has pages in our supplemental index, it does *not*
indicate that your site has been penalized.  In particular, we do not
move a site's pages from our main to our supplemental index in response
to any violations of our Webmaster Guidelines.

2) Freshness?
You can expect to see a fresher supplemental index in the coming
quarters.  By the definition of "supplemental," however, I don't forsee
it becoming as comprehensive or frequently updated as our main index.

3) Cure?
Get more quality backlinks.  This is a key way that our algorithms will
view your pages as more valuable to retain in our main index.

If you'd simply like to get some old pages OUT of our supplemental
index (and out of Google altogether), then you can use our automated
removal tool:
http://www.google.com/support/webmasters/bin/topic.py?topic=8459
Beware: Once removed from our index, the pages will STAY removed for 6
months and won't be reincluded during that time, so remove carefully
(and only use this tool if absolutely necessary).

Regards,
Adam