This one comes courtesy of dkp, but since he's being stubborn about posting to his blog, it has to appear here.
Find the file SmileyTable.plist nested way down in your /System/Library folder and rename it to SmileyTable.plist.bak
This will prevent iChat AV from rendering the smilies with images and will instead render them using text.
Because your change affects the entire system, you'll need to go into the Terminal and use sudo to mv the file. To undo the change, simply apply the instructions in reverse.