You can have iChat turned on, but be invisible to your Buddy list. That lets you know who is on, but protects you when you don’t really want to be interrupted. Sometimes you are just too busy to stop and chat.
Open your Buddy List and find your own name at the top. You will probably see the word Available with a small arrow pointing down next to it. Click on that and scroll down to the bottom of the options and you will see an option for “Invisible”. Click on it once and a check mark will appear.
As long as you leave it set that way, it will appear that you are not actively online in iChat.
To change it, just go back to that window and select one of the other options.