While 5 million pages is a very big number (that probably requires a lot more planning) this might be the Google document you are looking for:

http://support.google.com/webmasters/bin/answer.py?hl=en&...

  Moving your site
  If you're planning on moving your site to a new domain, 
  here are some tips that will help you retain your site's
  ranking in Google's search results.

Edit: http://www.mattcutts.com/blog/moving-to-a-new-web-host/ (2005)

http://googlewebmastercentral.blogspot.nl/2008/04/best-pract... (2008)