A previous hint mentioned how you could use custom emoticons in iChat by modifying SmileyTable.plist, but I have often been annoyed that when listing things in a chat (such as "A)... B)...") that iChat would display the "B)" as an emoticon. So, similarly, if you would like to disable emoticons selectively, you could do so in SmileyTable.plist or disable them entirely by renaming the file to SmileyTable.plist.bak. You will need to relaunch iChat for these changes to take effect. This has been tested on both iChat AV Public Beta and iChat AV 2.0.
[robg adds: You can find the SmileyTable.plist file in /System -> Library -> PrivateFrameworks -> InstantMessage.framework -> Versions -> A -> Resources -> English.lproj. I tested this and it does work...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031116210750208