good article by Jon udell at
http://www.infoworld.com/article/04/03/12/11OPstrategic_1.html
Consider S/MIME digital signatures. It's very likely that
your e-mail
client supports them.
But it's also very likely that someone's antivirus mail relay will
reject all mail with a .p7s signature attachment.
I don't think that very common. There are completely clueless
virus scanners arround though.
Maybe that's why eBay and Paypal aren't signing all their
outbound mail.
No, the user interface is the current issue. We are working on it.
In the meantime there will probably be a 'click here to opt to have
your email sent securely option'