For this script to work, both GUI scripting (under System Preferences -> Universal Access) and the iChat status menu item (iChat Preferences -> General -> Show status in menu bar) must be enabled. First, copy and paste this script into Script Editor. Save as an application and then add it to your login items. On startup/login, you will sign on to your iChat account without iChat launching.
As an added bonus, you can modify the script so you sign in with a status other than Available -- Use Away, or any custom status message you've created, or even Current iTunes Track.
[robg adds: See the comments to the referenced hint if you want some reasons why you might wish to sign on without having iChat active.]

