Very strange... I downloaded the Upgrade zip file, unzipped it on my test system, ran the upgrade, no problem...
When I opened Visual Studio and started debugging, I ran into this error... and since then, I have it. Inspecting the Object Browser, I did not find the namespaces DotNetNuke.Services.OutputCache and DotNetNuke.Services.ModuleCache.
Best wishes
Michael
UPDATE: I re-unzipped the downloaded file, started the application, and without running the upgrade procedure, everything worked fine. I started it again from Visual Studio - same errors. I unzipped the downloaded file, and searched for the file /bin/DotNetNuke.dll. It had a date/timestamp 11/25/2009 11:06 and displayed version 5.2.0.275 in the file properties. The file in my web had the date/timestamp 10/01/2009 13:04 and version 5.1.4.3. I just copied the newer file (from the file system to the web server's file system), started the application again in the browser - worked fine. I opened VS, checked the file with the Object Browser - both namespaces were there. I started it again from Visual Studio - and the file was overwritten with the old version again. The namespaces were missing in the Object Browser. I tried to import the file via Visual Studio, confirmed to overwrite the existing files - namespaces were there. I started from VS - which "restored" the old file and fired the error. I stopped IIS, deleted everything from the Temporary ASP.Net files, copied the newer dll again. Worked fine in the browser, but after starting the app from VS - same thing as before. This is really very, very strange...