OK, it’s kind of hard to explain so bare with me. You might have to try it out to completely understand this trick.. oh and the other thing, it’s completely useless but fun.
So type this in iChat to a friend:

/me thinks you rock

They will see in smaller gray type “your user name” thinks you rock- so in my case, “marilyn thinks you rock”. This shows up in the same kind of type that shows the occasional time and date stamp or “username” has gone offline. It appears this only works if both users are on Leopard’s iChat. The part that makes this work is the “/me” and you can type whatever you want after it.

I use it to send subtle messages to my husband- “marilyn deserves jewelry”