I don't know for certain but....
I would guess that the apps are tied to your bb id so if you put your new bb10 device on that bb id, then the PlayBook apps you bought will transfer over much as what happens when you upgrade your handheld device today. I doubt very much though that RIM will allow you to have the same bb id on both devices at the same time. This means, I fear, that you'll have to buy the apps again if you want them on both devices.
I'm not too concerned about this as many of the more expensive apps I've bought are games that I doubt I'd want to play on a 4" screen and I'm happy to support the developers who had made the bb platform a priority. Hope this helps.