Google rolled out its Vault secure email archiving service back in May, but at that time only new and very recent sign-ups to Google Apps could access the service. Now the Vault has been opened up and is available to all existing Google Apps customers.
Google Apps Vault costs $US5 per user per month, on top of the regular Google Apps charges. It creates a permanent searchable archive of all emails (including those which users might have deliberately deleted). The one group of customers who still can't access Vault are those who purchased Google Apps through a reseller rather than directly from Google; Google says it is working on a solution for that group.
Google Apps Vault now available to existing Apps and EDU customers [Google Enterprise Blog]