If you are doing development of browser-hosted .xbap WPF applications, there can be some confusion and delay when you want to clear out the cache. If you have the .NET SDK on the machine, you can run the mage program with the following switch:
I just learned a new trick for clearing the cache on a machine that doesn't have the sdk. Run the following from the command line:
rundll32 %windir%\system32\dfshim.dll CleanOnlineAppCache
This effectively does the same thing. In fact, poking into mage.exe with Reflector will reveal a reference to the unmanaged dfshim.dll.