I found this:

I'll let you know if this works
In AIM v5.5 you need to use RegEdit to navigate to HKEY_CURRENT_USER\Software\America Online\AOL Instant Messenger (TM)\CurrentVersion\Users\SCREEN-NAME\Misc and modify 'ShowAimTodayWindow' to 'DWORD:00000000'.
edit: replace SCREEN-NAME with your sn.