I had configured PKS (Podcasting Kit for SharePoint ) June 2009 release. Everything went smooth and even the application was working well except the encoding which i haven't yet tested .
Anyways after couple of days when i tried accessing the site the main page went KAPUT , i started getting this error "Unknown server tag 'asp"ScriptManager" .Now the question was "What might have gone wrong?" . I didn't even touched the site for 2 days and everything was working fine before (Typical sentence used by most developers -;)) .
After some search i found one article which asked to check the web.config file for some entries and it's weird those entries were gone from my web.config file. (Murphy's law in total effect ) .
Anyways as suggested i made the following entries ,In case image is not clear click on it to get better view.
I am glad website is back . Up and running .
Here is the link where i got the web.config entries .