If an existing web presentation is moved to a new URL scheme or domain, most people are unsure about their precious Google page ranking. Most sources agree about the following rules:

• Redirect every single URL by answering with HTTP error code 301 (Moved permanently). A 301 permanent redirect is not considered webspam by Google. Make absolutely sure that no 404 error (page not found) occurs somewhere. Do not perform wildcard redirects.
• Try to keep the content of old and new page somehow similar, in order to stick with the indexed key words from the old page.
• Invite GoogleBot by an updated sitemap file. If you have a sitemap for your old page, resubmit it, so that GoogleBot finds the 301’s earlier.
• It’s okay to have multiple pages that perform 301 redirects; but you should try to avoid multiple redirects one one URL (e.g. A -> B -> C -> D).

Sources:

http://www.mattcutts.com/blog/seo-advice-discussing-302-redirects/
http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=83105
http://groups.google.com/group/Google_Webmaster_Help/web/q-a-from-the-junetune-live-chat