Posterous launches easy post tagging via email
blog.posterous.com
blog.posterous.com
Send an email to post_tag1_tag2_tag2@posterous.com
I think the (Tags: tag1, tag2, tag3) syntax is a bit harder, though I have no idea if the other one would even be feasible (some clever virtual email address and parsing thing would have to be put in place, I guess).
Tagging has no relationship with the "to" field of an email. However, the subject line in an email is just that, the subject of your post. Which is a perfect fit for what tags are.
If there's better syntax than ((tag: we're open to change it. but it needs to be something that can't be misinterpreted.
daren.posterous.com