This really makes the case for much more public-key cryptography everywhere -- if all of the emails between HBGary, even internally only, were encrypted, HBGary would have gotten out with just a small DDOS and been meandering along just fine today. I think that people that run a computer security company should at least be able to figure out Enigmail.
This is what we do. We use Google Apps so we used a combination of existing policy, crypto and user awareness. It's not the use of email that's an issue, it's the how the data is stored. If it's encrypted with good crypto it's not a problem. If it's encrypted with bad crypto or no crypto then the extent of the problem is down to the data.
As an aside, while you would want to encrypt anything sensitive, that doesn't mean you need to encrypt everything - it certainly makes conversations over smartphones more difficult, and google chats wouldn't be encrypted.
Still, a little common sense goes a long way.