The Postini services freely bundled with Google Apps Premier
used to include a 90-day rolling archive, but it was discontinued on
April 22, 2009.
You can continue to purchase Postini archiving separately though (1yr retention costs $13 per user per year, and 10yr retention costs $33). But from what I recall you can't purchase it directly, you'd have to find a Google/Postini reseller...