i see people ask about when it's coming, but don't know what it is, or why it would be useful when/if unity supports it.
Currently if you place a prefab inside another prefab, the nested prefab is no longer linked to original. There are tons of practical uses, a good example would be UI. Say you create a button and make it prefab. Now you create a bunch of menus/windows/hud/etc that use that button. If you prefab those elements, you would have to manually update that button in each one. If you could nested prefabs, you could just update the original prefab and the rest will automatically have those changes.
Look at this Unite video from 2012 where Unity demonstrated an early prototype of some possible implementation of nested prefabs. Go to 59:30 into the video. http://www.youtube.com/watch?v=24AY4fJ66xA#t=3224
Let's say you have multiple prefabed characters, each with the same item as a child of the character. Under the current system you would have to update each character's prefab individually if you wanted to change the item's stats. With nested prefabs you could bury an item prefab in the character's hierarchy and then only have to update the stats once to apply it to all characters.
Check this post about my Nested prefab implementation, it's also allow to inherit prefabs... http://forum.unity3d.com/threads/236719-Prefab-Evolution-Plugin
We have a saying here in Sweden, Dont say hi (To your friend on the other side of the road) until you have crossed the road
The parenthesis is just to explain the idiom So it basicly goes, "Dont say hello before you have crossed the road" I guess you can get runover or something on the way over
I also found this one from the english language, don't count your chickens before they're hatched Or this one, "Dont let your design rely on nested prefabs before UT gets their ass out"
We have another saying, "Sit down in the boat". It's used when people are over hyped for something edit: Its hard not being over hyped, I see lots of workflows that will be so much easier with this