Scoble makes a good point about internal mail:

I left more than a gig of email at Microsoft. And that was after deleting all the crud out of it. What knowledge was in there? Tons of stuff about Channel 9 that would have been awesome for someone to use to learn about how things get onto Channel 9 and how it evolved. Gone. Deleted.

When an employee leaves, the silo that was their email is just gone. You'll never be able to capture all of that, but internal blogs and/or wikis could help. There are problems with that approach, of course - not everyone is going to be willing to do their information management that way. I suspect that it's something that ought to be looked at though.