For anyone coming across this now, BEFORE YOU DO ANYTHING ELSE… if you’re certain your classes/namespaces are correct and rebuilding has not solved your problem:
Try restarting Visual Studio
That’s it!
This appears to be a bug with Visual Studio 2012 (also appears to affect all other versions that support XAML development)
Update: If restarting Visual Studio doesn’t work, reboot the entire PC.
Update: As mentioned in comments by @Dunk, if restarting Visual Studio doesn’t work, try deleting the .suo file