VThis is very simple and just involves common sense.
Streaming works the same everywhere. IF will not only 'stream' only the terrain that's immediately in front of you (for example 5 miles ahead)... This obviously would mean you'd see blank space as you fly along. The app will obviously stream large portions ahead of you further than your eye can see.
As I'm not a developer, I don't know the exact distance / details. I don't mean to speculate, but the lack of common sense is astounding (not just you, but the 100+ comments all regarding the streaming aspect). Obviously if you are heading due East at 90 degrees, IF will stream the terrain that's a good distance in front you, I'm assuming something like 75 - 100 Nautical miles ahead of you. As you continue to travel, and the app steams the more terrain further east, the app can then empty the terrain that is now way behind you from the cache. This will allow for any decent 'drop out' of you wifi or 3G Connection, to not just freeze up and end your flight.
Look up what cache is, the devs have clearly stated the streaming data and terrain will be cached (and can be cleared). This means as you fly around, the terrain will be streamed and cached (stored on your device) in a big giant area around where y are flying. Streaming stays one step ahead of where you are. Then, if you do a 180 and turn around, the cached (stored) terrain is already there and you can just fly back through it and there is no need for further steaming at this time.
It will not take up much space on your devices. STOP ASKING if your certain device will have enough space. The app would simply stream and cache only a certain amount as it needs to 'write over the old terrain'.
The devs aren't morons, they know what they are doing, and they are also testing it out. It's actually a very, very simple system they intend to use, so stop whinging, and asking questions over and over again.
As for pricing, and 'oh I bought live+ last week blah blah how much will it cost or do I have to pay' ..... seriously, if only the forums had a 'report' button and we could ghost you after three.....