I recently found that if you have installed Adium on your Mac, it automatically becomes the default application to handle all the IM protocols it supports. This is really bad. When you install Adium, it will register all the IM protocols, without asking. Bug or not, the developers are working on a new option to handle this more gracefully.
A simple preference to change this, just as Safari lets you switch the default browser, should be built into iChat. But alas, Apple doesn't think so. And thus, Adium's assumption that you will use it for everything is troublesome -- how do you return to using iChat (AIM protocoal) once Adium has been installed? Considering Adium doesn't support even basic audio chats and has trouble with file transfers, this is a valid question.
However, you can easily set iChat as the default for AIM urls with More Internet. It's a preference panel that let's you change default programs for certain internet protocols. Although we shouldn't need a third-party application to solve this problem, it looks like it's necessary until iChat and/or Adium are modified.
[robg adds: The anonymous author didn't provide a detailed explanation of what to do with More Internet. I believe you simply need to add 'aim' as a new protocol, select it in the list, then click Change and point it to iChat. If I'm wrong, I'm sure someone will correct me, and then I'll modify this note!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006022018251177