Are you afraid of changing your domain because you're afraid that you'll lose your rankings? It's a valid concern and one that a Google Groups member is quite worried about. What's the best possible way to handle domain changes without any loss of rankings?
Two Google Webmaster Central staff members, Wysz and JohnMu, explain that the best procedure is to implement a 301 redirect. In May of last year, this question was addressed on the Google Webmaster Central blog.
JohnMu explains that it's best to "redirect incrementally" instead of doing hundreds (or thousands) of redirects at once. His reason is that you should be able to check that it's working properly before continuing.
A word of caution: don't use a 302 (temporary) redirect. What that tells Google is that you're saving the old URL. John explains that this can "confuse the crawlers." Finally, he mentions that he moved a site with about 100,000 pages before he joined Google and noticed no long-term negative effects.
While all of this is helpful, you can't help but wonder if Google Webmaster Tools should give webmasters the ability to specify that they're changing domains so that Google has some awareness before (or during) the process -- and I'm certain that it would alleviate some stress that is impacting these webmasters.
Forum discussion continues at Google Groups.