New message prefix preferences are in for the next binary! :-)

Message prefixes can now contain time stamps, specify alias/userName display, conditionally display additional text, contain inline messages, and support tail prefixing. When a prefix with an inline message is used, the message view will hide user icons and bubbles, reverting back to a classic IM look.

It's pretty spiffy stuff, and more powerful than the preferences let on. :-)

Because of these changes, you must either:
A) User the message display preferences to select a prefix from the menu
B) When Adium is closed, trash the file located at (From your home folder):

~/Library/Application Support/Adium 2.0/Users/Default/Message Display.plist

to reset your prefixes to the new defaults.