Hi, I'm developer of offlineaddress.com.

To be able to develop secure bitcoin services we need a way to allow developers to prove security of their sites, without hurting usability for average bitcoin users.

Validating service could be generalized and also useful to sites that aren't related to bitcoins.
It might sense to contact:
 - GitHub.com
 - EFF.org
 - TorProject.org
because it seems to me that they might want to solve similar problems.

Solution could be:
 - third site, as described by canton (general, or specific to bitcoin sites).
 - browser plugin that compares checksums automatically on each visit, for sites with specified meta tag with link to the source.