Well, all photos that you take with your device are in Photo Stream (in the cloud) for a month. So they are in both places (the device and the cloud). The only other photos you will see in Recently Added are photos taken on other devices that are in Photo Stream.
We don't have a lot of info about this at this point, but here is how I'm thinking it will work (and for the sake of simplicity, we will talk about one device):
Before, when you took photos on your device, they showed up in Camera Roll and in Photo Stream
After 30 days, the photos were deleted from Photo Stream in the Cloud, but if you did not turn Photo Stream off in the interim on your device (which you wouldn't normally do), then your Photo Stream on your device could show up to 1000 photos. However, if you did turn Photo Stream off and then back on again, only the photos for the current month would reload into Photo Stream (because that was all that was still in the Cloud for Photo Stream)
Camera Roll photos would stay in the Camera Roll until you deleted them
I'm thinking that now, the Recently Added photos will hold both Camera Roll and Photo Stream photos for the current month (or the last 30 days), and then they will be moved into the the Albums with a separate tag for each date on which you have taken photos. The Album view hierarchy is:
Years
Collections: which has photos grouped by date ranges (not sure how the grouping here is determined)
Moments: which has photos grouped by a single date (all photos taken on that date)
What I'm not sure of is whether or not all photos that are not imported will still show up to be imported even when they are no longer in the Recently Added folder, but I'm guessing that even when they have been moved out of the Recently Added, they will still import if they do not exist in the photo library when the device is hooked up.
So, as you can see, a lot of this is, at this point, just surmising, and as we go, we will be able to either acquire or will ascertain more exact info on how this all is going to work. But hope that makes the structure a little clearer for now!
Cheers,
GB