Input the code below into Script Editor, and save it as Auto Accept modified.applescript in /Library » Scripts » iChat. Then, in iChat's buddy list, control-click on the name of the person from whom you'd like to auto-accept screen sharing, and select Show Info from the pop-up menu. Click on the Alerts tab, and set the Event pop-up to Audio Invitation (screen sharing comes in as an audio invitation). Next, click the Run AppleScript check box, and use the pop-up menu to select the script you just saved. It's very important that you enable this on a per user basis, not through the Alert tab in iChat's preferences -- unless, that is, you want all your buddies to have full access to your computer without your confirmation.
The same can be done for Video Invitations, Text Invitations, and Incoming File Transfers, respectively, as they're all covered in the above script.
[robg adds: I tested this, and it works as described...]

