Ok, I am going to summarise my experience here as it might help others who are upgrading. My app uses the Telerik RadgridView control and Forms authentication. It runs OOB:
1) Firstly, I upgraded to the RTM LS2012 version and rebuild my app - all works fine on Dev.
2) I then tried to deploy to my Win2008 Server with no changes (note: it was running the LS2011 version with no problems). This failed with an "Unknown" message during the publish process. To get around this I unchecked the flag on the Publish wizard "IIS Server has the Lightswitch Deployment Prerequisites".
3) Next problem: The Silverlight App would show "loading" and then show a blank screen. At one point I received a message saying "Debugging resource strings are unavailable". The problem here was that the IIS site that the publishing wizard created had Windows and Forms Authentication (under the IIS Authentication icon). Disabling Windows Auth solved this problem. I was finally able to log on to my app.
4) Hardest problem: The app would log in and work fine for about 20 seconds. Then it would throw a Silverlight unhandled exception: "Unhandled exception at 0x5ceed700 in sllauncher.exe: 0xC0000 094: Integer division by zero". After a lot of Googling someone asked if I was using a VM - the server IS a VM and they suggested I run as a single processor (ie. it was a threading error). I did not try this, instead I tried running from a different PC as a client OOB (I was testing on the server). This then worked.
After all this my app is now up and running as Lightswitch 2012.... hope this helps someone!
Notes: I did NOT install .NET 4.5 (my projects target .NET 4) OR the Lightswitch 2012 requisites on the server. It works with the Silverlight 4 Telerik dlls.
