I use iChat for two accounts of mine, one for GMail & another with Yahoo!
After some while using ichat I found that iChat annoys you with asking for your respective account password again & again… Even if you have stored the same password in keychain.
But I have found work around for this problem…
While holding the ‘Option’ button on keyboard go to Finder’s ‘Menu Bar’ click ‘Go’
You will see your library folder,
Delete Cache folder from that Library folder, then after rebooting clear Trash
Now, Use Disk Utility to Verify & Repair your Macintosh HD’s Disk Permissions…!!!
Your Problem with iChat will be solved…
Thanks & Regards