An interesting WebmasterWorld thread discusses the phenomenon of redesigning a site, with brand new URL structure, 301 redirecting the old URLs to the new, and seeing an almost immediate boost in rankings at Google.com. In fact, WebmasterWorld moderator, minnapple, has coined this phenomenon the "honeymoon period."
What he means by that is that sometimes, a well-liked site, will see an immediate boost in rankings at Google after redesigning and implementing many 301 redirects. Minnapple described the honeymoon period as lasting about 20 days, and it is possible things can go back to how they were, or possibly get worse.
Tedster said he has seen similar things:
This lines up with the results of a redesign I helped on - we saw the new urls take over seamlessly. I'd say this is a sign that you had things well handled technically, and that Google already liked the site a good bit. Please let us know if things start to wobble.
Have you experienced this Google redesign "Honeymoon Period" yourself?
Forum discussion at WebmasterWorld.