Max Masnick, arguing that Google reading your emails should be the least of your concerns:
Email is simply not secure. Messages are not secure in transit: the protocol used for sending email (SMTP) does not require encrypted connections, so it could easily be intercepted by a third party as travels over the internet. You also have no guarantees about the security of a recipient’s email client or server.
He makes great points, not to mention that there’s nothing stopping the recipient from posting your email on thier blog. However I don’t think that all the other security risks associated with email should make you dismiss the privacy invasion you grant Google when you use Gmail.