I don't see how you could possibly implement proximity sensing without background processes, and without draining the battery. However, it would be tremendously useful if agenda items could be tied to the corresponding contact's address book record. A few ideas:
- Add items to the user's notes. I did this via AppleScript a while back.
- Add a badge to the user's avatar to indicate that there are action items associated with the contact record.
This way, when I get/place a call, the actions (or at least a visual indication of the presence of actions) associated with the contact are right in front of me.