This seems like a simple question, but I'm having trouble finding out the answer. I'm building a map in tilemill and using data from a geojson file. I plan on hosting it later using tilestream. Once I do, will I be able to update the underlying data and have it be reflected in the map? Or will I need to reexport and deploy again?
Currently you'd need to reexport and deploy. This is what we do for some sites with moderate (weekly, sometimes faster) update cycles. It's certainly planned to do more live rendering in the future, but we aren't going to recommend anything until it's actually fast enough and stable enough to keep up with widely-deployed sites (a criteria that few systems actually hit).