In the above example, I'm posting to both Twitter and Posterous (but not Facebook). If I wanted to autopost to all 3 (having previously set up my Twitter and Facebook credentials), I could just send the message to post@bnyce-audio.posterous.com.
My set up is slightly more advanced, in that I have more than one posterous account. Otherwise, I could have just mailed the message to post@posterous.com (without the prefix "bnyce-audio").
In the subject line I've identified some tags. Note the syntax: "((tag: apple,banana))".
The mp3 file is added as an attachment.
Easy!
mp3splt -f -g [@o,@a=Terrence\ McKenna,@y=1994] 201-McKenna-ImaginationPt1.mp3 2.49 3.58