There isn't any built in method to archive messages to a second tier storage solution. Exchange 2007 does have MRM which allows users to catagorize email so that retention rules can be applied to it. It is a start but not an end all solution for most companies.
There are a number of products that can do this like Symantec Enterprise Vault or C2C Policy Manager. I am a big fan of disabling PSTs with a GPO and enforcing limits, but that doesn't work out so well :)