iBeacon is a little bleeding-edge right now, particularly since it requires a hardware investment, but I can definitely see the use for it as the technology becomes more popular and mainstream.
In the shorter term, I think it would make more sense to use Bluetooth proximity going down that road. My car and my home already have Bluetooth devices that can be used to detect proximity. Even my door lock is Bluetooth in fact (although I’m arguably in a minority there :) ).
Time-based contexts for location notifications would definitely be great, and arguably much more useful now that OF is actively pushing out notifications. Being reminded to “Buy Milk” when I drive by the grocery store at 3 AM isn’t really all that useful – it’s more of a distraction than anything else. It was one thing when I was actually looking at a Nearby list and mentally filtering, it’s quite another when my iPhone is buzzing at me to do things that are essentially impossible at that moment in time.