Hi
From what I have seen, if the NEW structure is done well (good architecture, good on-page optimization, better navigation, better keyword optimization in the new urls - its all an improvement from the old navigation) and you 301 everything corectly - you should see an improvement in rankings. Although this can take a week or two.
And of course submit a new xml sitemap. I also block the old pages/directories in robots.txt (this may be a little overkill but I do it anyway).
One extra tip is to just do a site:www.mydomain.com search in google to uncover any pages in their index you may not know about, or have overlooked and be sure to 301 those to the most relevant pages as well.
-Dan
