I stumbled across a company that's new to me: Voltage Security, who have recently announced a security plug-in for Windows Messenger. The company's technology -- for both IM and email -- is based on Identity Based Encryption, described breifly in a Terence Spiesarticle:

"The biggest drawback of PKI, aside from simply getting people to use it, is the rigmarole of generating and distributing public keys. One potential solution was proposed nearly 20 years ago. It's called Identity-Based Encryption.

The insight behind IBE is that public keys don't have to be large prime numbers. They could be email addresses, phone numbers, or even subject lines. Bob can send Alice an encrypted email before she's even generated her private key, before she's even heard of private keys."