[whpalmer4]

That would be nice, wouldn't it? It's a bit expensive to do, compared to what is offered now. Instead of just having a fixed set of targets whose proximity needs to be monitored, OmniFocus would have to frequently recompute the set of targets to be monitored as it noticed you moving around. In areas where cells are larger, it might not be as quick to notice that your position has changed, assuming that it is using the cell handoff instead of GPS receiver to cut power consumption. And of course if you head into an area without cellular data coverage, it may not work at all, whereas it might still work for the fixed targets (whose identity is already known), assuming location services drops back to using the GPS receiver.