Serena, I've wondered this myself. For WP sites, I found this information awhile back:
http://www.cindyalvarez.com/data-dri...wordpress-blog I think part of the key here is to use canonical links to avoid the duplicate content issue.
And related to the above link is this:
http://www.dellanave.com/blog/2009/0...heme-w-plugin/
Not sure I helped...but had to give 'er a try.
For non-WP sites, to me, it is harder to do and not get in trouble with Google by using cloaking or redirects, etc. Using a system (A/B splitting) whereas site A with changes is up for a while, and then replace with site B, may be an option, or even internal program logic, but I need to learn way more about this to make an informed decision.
Interesting topic - glad you brought it up!