How to remove a webpage from Google's index (cache)

This is the area that describes some of the options to keep webpages off the Google Index/Cache:
https://www.google.com/accounts/ServiceLogin?service=sitemaps&continue=https://www.google.com%2Fwebmasters%2Ftools%2Fremovals%3Fhl%3Den&ltmpl=urlremoval&hl=en

To request a URL removal, start here:
http://www.google.com/support/webmasters/bin/answer.py?answer=61062

And, here is a link to the Google Webmaster Blog that could be a good starting point:
http://googlewebmastercentral.blogspot.com/2007/04/requesting-removal-of-content-from-our.html