When you first see the options for creating a profile on iChat for Mac OS X, it seems like only plain-text is allowed. — Compared with AOL's own clients, this would be a huge downgrade.
However, rich-formatting is actually just a keystroke away. 🎨
Just like with chat windows themselves, by pressing “Command (⌘) + T” on your keyboard, you can access options for fonts, colors, symbols, and more! — There's actually a menu option (within the “Format” menu) to access this dialog as well, but since it's not as discoverable as in the official AIM clients, I think most people just assume that it doesn't exist here at all.
Try it out! 😎