While using the 'Explore' part of Posterous.com, I noticed that a lot of people, myself included, were using the default Posterous favicon. I assumed that we weren't given a choice until I saw Tweetdeck's Posterous favicon. Did they get an exemption? No. They made use of Posterous' option to perform advanced customization.

Here's what you need to do to set your own favicon:
1. Log into posterous.com
2. Scroll down and click 'Theme My Site'
3. Click 'Advanced'
4. Click 'Expand'
5. Remove this line:
    <link rel="icon" href="/images/favicon.png" type="image/x-png"/>

6. Place a line like this where the one you removed was:
    <link href="http://www.unixsysadmin.org/favicon.ico" rel="icon" type="image/vnd.microsoft.icon" />

7. Click 'Save, I'm done!'
8. Click 'OK'

You'll be returned to your blog. Note the favicon! :+)!