Many agencies today still live in the reactive state, as I call it, when it comes to SEO. No agency is completely immune from this, but others are almost thick-skulled, it seems. When you go about a site redesign: remember the search engines! If you get a fair amount of traffic from search, and then revamp the site without thinking about any rankings implications, you may find yourself suddenly "invisible."
Many threads can be found each day dealing with the subject of 301 redirects as well as about ensuring your site statistics counters are properly configured. A recent thread at High Rankings Forums showed what can happen when a site is redesigned and a potentially jealous former developer looks at what he feels are your stats. The thread, titled "Can Traffic Really Drop By 90%? A sanity check before talking to client."
Enjoy the thread and remember SEO or at least proper redirecting should be "baked-in" to any site redesign. Be proactive!