We could do it that way if we wanted to, but it sorta doesn't make sense. The "to" address determines where you are posting "to". We already use this for things like flickr@posterous.com to indicate you want your post on flickr. And soon we'll do post@sachin.posterous.com so you can post to various sites.

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.