You can make an empty commit with just a message. Kind of like git-tweeting.

Add this to .gitconfig:

[alias]
  msg = commit --allow-empty -m

Then add your message:

$ git msg "Ryan sucks."
[master e3900c8] Ryan sucks.

Now everyone will know Ryan sucks.

$ git show
commit e3900c87e0c7928d071feb032ac38ab5712bfa04
Author: Chris Wanstrath <chris@ozmm.org>
Date:   Mon Nov 23 12:33:13 2009 -0800

    Ryan sucks.

Hat tip to rtomayko for showing me this.