In one of the detailed Google Popular Picks thread, Googlers discuss the issue of redirects.
In March, the Google Webmaster Central team talked about moving your site from one location to another. In this post, however, JohnMu gives more insight onto how you should work with redirection.
If you move your site or domain and change the structure, a 301 redirect is still the preferred tool for notifying servers about the new URL destination. If you moved specifically from an old domain to a new domain, it's helpful to use this operator:
http://www.google.com/search?q=site:old-domain.com
where old-domain.com is your old domain. That way, you can see how many pages Google has in their index of your old domain only.
When you consider moving your site, you should also look into addressing duplicate content through URL parameters as the Google Webmaster Central blog discusses.
Finally, what about when you move your URLs and you have absolute URLs in the paths? That's the preferred method, but Google can parse through the relative URLs as well.
If you have any questions or want to chime in on the discussion, the Google Groups thread is still open.
