I am currently developing two separate locative content systems. Meaning, what you see is determined by your location. We map your IP address to a geocode, or latitude and longitude. Hopefully in the future geo headers will be added by browsers and we will not need to play this guessing game.
Looking through the logs and seeing the regular list of search engine indexers I normally see, what comes to mind is that the engine will see locative content based on its IP address. But the rest of the content will not get indexed unless we make sure to give them a way to find it.
I suppose that is a long way around of saying sitemaps are important after all, though I would have normally made then very low priority.