In the OS X 10.5 version of iChat, you can easily move between buddy lists windows for various accounts—such as your iChat account, your Bonjour account, and maybe a Google Talk account—by using the Windows menu. There you’ll find Command-key shortcuts for each active account; Command-1 activates the first account’s buddy list, Command-2 the second, and so on. But if you’ve checked out iChat’s menus, you knew this already.
What you may not have known is that you can control the order in which the accounts appear in this list. So if you use Google Talk most often, you can make its shortcut Command-1, and relegate Apple’s iChat buddy list to Command-2 or Command-3. To change the order of the items in this menu, open iChat’s preferences (Command-, [that’s Command-comma]) then click on Accounts. In the list of accounts on the left, just click-and-drag to reorder the accounts however you wish. When you have things ordered as you wish, just close the preferences window, and you’re done.
Check the Windows menu now, and you’ll find the account you moved to the top of the list is also shown first in the buddy list section of the menu, and has been given the Command-1 shortcut.