You can modify the theme to include the timestamps. It is just a matter of file main.css and updating the .contenttime and .time rules. For example, I have:

.contexttime, .time { display: inline; font-size: 80%; line-height: 18px; color: silver; }

File:

~/Library/Application Support/Adium 2.0/Message Styles/Dinosaur Comics.AdiumMessageStyle/Contents/Resources/main.css