We don't know the exact details yet behind this, but you will only see 1 continuous library, of which will be completely available in iCloud and all your devices. Maybe the phone will store all the originals until it runs out of space (and then starts to delete the oldest ones locally - but still available in iCloud and still visible on the phone and all devices), OR maybe it'll keep only a specific amount cached locally, and slowly replace the local ones with phone optimized resolution & size but full size & resolution available in iCloud. Basically this is still unknown.
When iCloud Photo library is enabled, you no longer have a Camera Roll. iClould Photo library will be the primary location of all the photos you take and will be kept in iCloud as well. Considering this, I think it's safe to say they won't store them in your iCloud storage through the iCloud photo library and then again through iCloud backups. I would guess that when doing a backup you won't see photos as an option as they'll already be in iCloud.
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Honestly, this is going to be such a nice solution to photos. My iPhone only has so much space to store photos, I usually connect it to my Mac once every few weeks to copy all the photos off, sort them into albums and events in iPhoto, but it becomes such a manual process, and is really starting to feel archaic. Now I have a photo library isolated to iPhoto on my Mac, and not easily visible anywhere else. The beauty of this iCloud photo library is that all photos & video you take will go into one continuous stream that is synced everywhere (iPhone, iPad, Mac, iCloud web). All content will be visible everywhere. No more plugging and manually importing photos to iPhoto and deleting them off the device. Now I'll be able to pick up my iPad, sort some recent photos I've taken into albums, and they'll be visible like that everywhere (iPhone, iPad, Mac, web).
