The newest version of iChat on Leopard (4.x) has a nice new feature that is already available on many other IM clients - you can now be invisible. If you set yourself to be invisible, you can see all of your friends and chat with them, but until you start chatting with someone you appear to be off-line to them. This can be really useful at cutting down on interruptions.

To set yourself as invisible on iChat just click on your current Status to bring up the Status menu and then look towards the bottom. You'll see the "invisible" status down there.