First off, a disclaimer, i work for Global Web Security Systems (gWebs).
Our software grabs outgoing and incoming data at the transport layer as you use google products and tosses it through GnuPG.
Our MailCloak product encrypts gmail, (Yahoo, and MSN, etc) in firefox and IE.
DocCloak, in private beta, will do the same thing for Google Docs, and Zoho office.
SaaSCloak, again in private beta, works with google sites, and we are adding several other cloud services.
Check this stuff out at http://www.getmailcloak.com/
to install MailCloak
to learn more about the company.