I ran into a minor problem with an existing web site I have which was running Orcas Beta1 after I upgraded to Beta2.
It's a content rich site so I spend quite a lot of time just editing pages in between doing actual builds and while editing I kept seeing the error "Unrecognized tag prefix or device filter 'asp'." :
I couldn't figure out what was wrong so I ploughed ahead with my edits but eventually wanted to build and at that point found, not surprisingly that it would not:
So there I was staring at this error wondering what it meant and finally noticed the error below it - "Could not load file or assembly 'System.Core, Version 184.108.40.206". It often happens that one error in a build is actually the consequence of another error so if you cannot fix one error look at the others and see if you can fix those.
Double clicking the error took me to the offending line in web.config:
Ok, so Beta 2 installed ok so surely I've got System.Core.dll? A quick browse to the Global Assembly Cache (GAC) :
revealed that System.Core was indeed installed:
But look at the version number - 220.127.116.11.
There's the problem, we've revved the version numbers between beta1 and beta2 from 18.104.22.168 to 22.214.171.124.
Turned out that the same is true for System.Web.Extension.dll. This was referenced in multiple places in my web.config so a thorough search and replace was necessary.
Once the version numbers were all changed everything built and the mystery "Unrecognized tag prefix or device filter 'asp'." error disappeared as well.
Hope this helps!