I have dialup, so I am not connected when my computer starts up. The next time I connect, my iChat menu extra does not automatically log me in to the iChat network (it is still gray, and I must manually select "Available"). However, if I have opened iChat or the iChatAgent, I will be automatically signed on to the iChat network whenever I go online with my modem. This especially applies if you have an iBook that is not always plugged in, even if you do have a LAN or cable modem.
It is possible to sign on automatically to iChat without opening iChat itself. Instead of making an Applescript as suggested by
this hint, you can accomplish this task with just a simple login item. Browse to /System -> Library -> PrivateFrameworks -> InstantMessage.framework, and drag the
iChatAgent application to your Login Items in the System Preferences. By doing this, the iChat menu extra will automatically become active when you log in or go online.