RIM currently doesn't allow developers access to the bridge (PIM) data on PlayBook. Until they do, you won't see any task list apps on PlayBook that can talk to the database on the phone.
I have been looking for what you are - a good tasks app for PB that can at least back up to a file off of the device. TaskMemo appears to be the closest in concept, but doesn't quite meet my needs yet.
I have been in contact with Antonio, the developer of TaskMemo. He is considering developing TaskMemo Pro which may include more heavy-duty features like prioritization, complete category customization, projects, etc. I'm not sure when that would be, if at all. I believe he is a one-man shop.
There is also Conqu, but I found it to be too many steps to add tasks. It's pretty heavily tied to the GTD method too, so it looses a bit of flexibility there in my opinion.
It would be nice if RIM wasn't so stingy with features in their native Tasks app. The native database includes category and memo info - they just don't use it in their own app! Plus you can't filter or sort. Pretty poor excuse.