This would include encrypting instant messages. This could be done in a manner similar to encrypting emails. (see PrivacyManagementInEmail) However, it would need more support from the instant messaging client. I'm not aware of any instant messaging clients that currently support encryption.

Alternatively, if one is using a chat program such as IRC or even BeShare, it would be desirable to be able to encrypt the chat. This could be more difficult for an application to implement, however it would probably not require more support from the operating system perspective. Encrypting a sentence or two isn't fundamentally different from encrypting an email.