From my understanding bb10 apps don't work on the pb until it is updated. As for how easy it would be to make a current pb app and turn it into a bb10 app I don't know.
If developers aren't dumping BB and are actually developing for BB10 - then where are all these apps for the Playbook? The differences between the current PB OS and BB10 aren't that great that a developer couldn't make it work for the Playbook - especially since the tools are so easy to use.
PlayBook doesn't support Cascades, so if you're making a native app you have to design and create your own buttons, textfields, radio boxes, etc. Since it's a ton easier in cascades, developers just use Cascades and will port their BB10 apps to PlayBook when PlayBook supports Cascades. I have no idea when that will happen though. I heard that it was going to be in 2.1 a while ago, but I don't think it's in the Beta of it, so it probably won't be in the release version.