It isn't uncommon to see a temporary drop in rankings after a site redesign, i've seen it take anything around a month for the rankings to return.
In my opinion, changing back to the old site design/structure should be the last resort, as if you do this, you will be nervous of ever changing it again, and no good can come of that scenario!
As the site structure has remained identical, you should concentrate on the on page optimisation.
As you are a pro member, i would use the SeoMoz to check some of the basics.
- Set your old site up under another url, preferably on another server, make sure you no index, no follow the site as you don't want it being picked up by the bots.
- Set up the "old" site in SeoMoz's web app with exactly the same keywords as you have set up for the new site
- Run the on page reports for the keywords in which your rankings have changed on the new site and old site.
- Compare the findings, fix any problems on the new site.
Secondly
Once the full crawl has been done, check through the site errors (duplicate page titles, duplicate content etc)
As already said, the rankings dip is not uncommon, but the above will help you confirm that there is nothing wrong with the site.
Hope that helps and good luck!
