If, like me, you happen to encounter the following user experience when attempting to edit a master page...
...then follow these steps to resolve the issue:
- Start Control Panel and then open Administrative Tools.
- In Windows Vista, right-click Microsoft .NET Framework 2.0 Configuration and click Run as administrator. If you are not using Windows Vista, then right-click and select Run as... and specify an account that is a member of the local Administrators group (because you don't normally login with an administrative account, right?). Enter the appropriate credentials to start the configuration console.
- In the .NET Framework 2.0 Configuration console, in the tree on the left, expand My Computer, and click Runtime Security Policy.
- In the Code Access Security Policy window on the right, in the Tasks section, click Adjust Zone Security.
- In the Security Adjustment Wizard, ensure Make changes to this computer is selected and then click Next.
- Select Local Intranet, in the Choose the level of trust for assemblies from this zone section, click and drag the slider to Full Trust, and then click Next.
- Confirm the setting in the Summary of changes and then click Finish.
Restart SharePoint Designer and you should be "good-to-go."