I'm writing this down because (1) I've had to figure it out a few times already and don't want to figure it out again, and (2) it may save you some snortin' and cussin' if you run across it yourself. It is astonishingly annoying.
If you have an MVC 3 project in Visual Studio 2010 and while refactoring you move a file - for example you move a view model to the shared area, you may suddenly encounter a compile error in some random temporary file like the one below:
The type or namespace name 'xxx' does not exist in the namespace 'xxx' (are you missing an assembly reference?) c:\Users\[user name]\AppData\Local\Temp\Temporary ASP.NET Files\temp\eab6c63b\948de17e\App_Web_hadnllup.0.cs
Cleaning the solution and/or deleting the temporary files will not resolve the problem. This can happen whether you simply drag and drop the file to the new location, then change the namespace yourself, or if you right click on the file and choose Refactor > Move and have Visual Studio move the file and update the namespaces for you. The latter process (in theory) will update all references to the file within your project automatically, while with the former, you would typically try to compile the project and then fix all the broken items that now appear in the Error list as a result of the namespace change. However, unless you do a global find and replace, you will probably still end up getting the cryptic error above because chances are that the @model declaration in one or more of your Views is still pointing to the old namespace for that viewmodel file. Update the view(s) and the error will go away and all will be fine.
Now, if you use SVN, things are a little more complicated: If you move the file using either of the techniques described above, what will happen is SVN will delete the original file and create a new one for the new location, which is fine, unless you were hoping to preserve the full subversion history of that file. If you want to preserve the file history in SVN and move the file, this is how you do it: in Windows Explorer, right-click and drag the file from its old location to its new location, then select "SVN move versioned item" from the context menu. This will not only move the actual file itself, but it will also make sure that all the file history stays with it after you check in your changes. Back in Visual Studio, use the Solution Explorer in VS2010 to "exclude from project" the (now-missing) copy of the file in its old location, and then "include in project" the file in its new location. You may need to refresh the view in solution explorer and/or make sure you are viewing all the files by clicking the "Show all Files" icon at the top (next to the refresh icon) in order to see these files.
After you update the namespace to reflect the new location, I recommend using a global find and replace before you try to compile to save yourself a lot of trouble.
To summarize, if you find yourself getting obsolete, broken references in auto-generated files that you can't permanently delete, look in your Views folder for the bad references.