Note for users on IIS7 in Integrated Pipeline mode
In your case you're using the new segment system.webServer in the web.config. If you already copied over the parts from system.web for a previous version of DMX (there are 3 modules and 4 handlers defined) then you'll get a site error after upgrade to DMX 5.1. Don't panic. All you need to do is replace Bring2mind.ComponentArt with ComponentArt in the web.config wherever it is mentioned. But make sure the old dll has been replaced with the new.
Technical details:
The modules and handlers tell IIS what code to run upon a web request. The modules run on every request. These modules include 2 modules for progress handlers of upload components for DMX. One of them uses ComponentArt. Since the namespace has changed for v 05.01.00 it means IIS will no longer find that bit of code and it will assume the site is broken. What needs to be done is to inform IIS that it is now a different namespace (i.e. the one without the leading Bring2mind).
The namespace Bring2mind was introduced at first to keep the dll apart from other vendors. However, DNN is now making moves to provide proper handling of shared libraries. DMX 5.1 is preparing for this future and the dll is moved back into its old namespace. A second issue was that there were issues with the custom namespace in .net 3.5.