It’s official now, Google Vault which does eDiscovery for Gmail is now now available to existing business customers. However, it is not yet available for Government customers.
See the announcement from Google here.
After enabling Vault on the Google dashboard, you can log into ediscovery.google.com and use it to place legal holds.
Another feature that Google Apps has is the ability to make custom internal websites with forms to distribute to users that can track and specifically require acknowledgement from custodians about the legal hold and need to preserve data.
Enterprises use product like Legal Hold Pro to do this, and this type of functionality can be produced inexpensively and quickly with Google Apps Sites and Forms.