Some law sites suggest that if you do have a message you need to send of a sensitive nature that you use a manual disclaimer, encrypt the message and then add the message itself as an attachment to the disclaimer. This still is probably not legally binding, but at least it gives the impression that the recipient read the disclaimer before they opened the (sensitive) attachment.
If you are considering using an email disclaimer, please discuss the topic with a lawyer before making a decision either way.
Below are a few additional resources you may find useful:
An entertaining look at the “stupidity” of email disclaimers, with plenty of examples: http://goldmark.org/jeff/stupid-disclaimers/
A legal take on email disclaimers: http://www.weblaw.co.uk/articles/faqs_on_emails_disclaimers_and_the_law/