If, like me, you happen to encounter the following user experience when attempting to edit a master page...

Figure 1: SharePoint Designer - "Error Creating Control"

See full-sized image.

...then follow these steps to resolve the issue:

1. Start Control Panel and then open Administrative Tools.
2. 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.
3. In the .NET Framework 2.0 Configuration console, in the tree on the left, expand My Computer, and click Runtime Security Policy.
4. In the Code Access Security Policy window on the right, in the Tasks section, click Adjust Zone Security.
5. In the Security Adjustment Wizard, ensure Make changes to this computer is selected and then click Next.
6. 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.
7. Confirm the setting in the Summary of changes and then click Finish.

Restart SharePoint Designer and you should be "good-to-go."