Google Code already has an option to flag a project as moved. From your project: Administer > Advanced > Project Moved
For an example of what this looks like: http://code.google.com/p/iosched
For an example of what this looks like: http://code.google.com/p/iosched
I work on Google Code, and we will be putting a service in place to redirect
deep links to project homepages, issues, etc. to their new locations.
Projects on Google Code will need to set the "project moved" flag, under the
advanced tab of the project. But once set, things should work like you
expect.