Actually I think my problem was some rogue copies of the Beta MVC DLLs hanging around.
I deleted them all, uninstalled the RC1 and made sure they all left the GAC and then reinstalled the RC1. So far everything seems fine.
Microsoft have now released a hotfix to resolve this issue.
See https://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=16827&wa=wsignin1.0
Phil Haack elaborates further here - http://haacked.com/archive/2009/03/06/hotfix-for-installing-aspnetmvc.aspx
Try removing all bin/obj directories, and clearing your Temporary ASP.NET Files and %TEMP% directories. Then issue the following commands from a VS2008 command prompt:
ngen /delete System.Web.Mvc
ngen /delete System.Web.Abstractions
ngen update
Also ensure that all your references (MvcContrib, anything else built against MVC) are pointing to the same version of MVC as all the others.
This seems to have worked for me (so far)
There are some framework bugs that affect all VS add-ins etc if they reference System.Core v3.5. Start by clearing out the NGen cache. "ngen update", "ngen /delete [assemblyname]" or a sweeping "ngen /delete *" usually does the trick
More details + workarounds for this (and/or similar) issues here:
http://forum.huagati.com/topic5-addin-causes-ide-to-close.aspx
http://code.msdn.microsoft.com/PowerCommands/WorkItem/View.aspx?WorkItemId=8
http://www.jetbrains.net/devnet/thread/274657
Update: finally someone from MSFT acknowledge that there is a problem: http://blogs.msdn.com/jnak/archive/2009/02/15/potential-crash-in-vs-when-using-the-mvc-rc-on-windows-azure.aspx
Update 2: An attempt at a workaround (VS2008 add-in): http://www.huagati.com/ProjectLoader/
Update 3: Microsoft has a CLR patch (KB963676) that fixes this problem. It is not available for download from microsoft.com but it can be requested through MSFT support / PSS.
Update 4: The CLR patch is now available for download from Microsoft Connect:
https://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=16827&wa=wsignin1.0
http://blogs.msdn.com/jnak/archive/2009/02/26/fix-available-asp-net-mvc-rc-crash-in-a-windows-azure-cloud-service-project.aspx
I have found that any compile issues with the master page or the page itself -- even warnings -- can cause this to happen. So close the project, delete the bin and obj directories, then re-open the project. Next open your master page(s) and any other recently changed aspx/ascx files. It is important to open all before you compile.
Now, viewing each page one at a time, compile the project and resolve the warnings. Once all the warnings are resolved, close the pages and try to re-open them.
The problem was indeed, powercommands for VS 2008. Uninstall them if you can live without them and the aspx pages/designers will open fine.