Just read an article on The Register about Microsoft's e-mail retention policy. Apparently, through several means, they have "encouraged" their employees to delete all e-mail older than 30 days.
As a working engineer, this policy seems barely short of insanity. I archive all project-related email I have, and I have email dating back 3-4 years for some projects. This is invaluable in many cases, when you want to find out what requirement somebody e-mailed you 9 months ago about such-and-such. Many of these issues and requirements are never tracked anywhere outside of e-mail.
Now, it's possible that Microsoft simply has much better project management than where I work, and that they track all of this externally, so retaining this isn't required. You've used their software, you make the call. Saving disk space? Google is willing to give you a gigabyte of mail storage for free -- deleting mail older than 30 days seems overzealous even for corporate frugality.
On the other hand, if someone sues you for anticompetitive behavior, all the e-mail evidence they would use against you is now, conveniently, gone. Oops! Sorry. This is not a company that was convicted of anticompetitive behavior and reforming -- this is a company that got caught and is making sure it doesn't get caught again.
Update: Bob Cringely has an article with some details as to how this curious document retention policy came to light.