I imagine that we all have backlogs of work we would do if only we had time. So do you keep them in your someday/maybe list? To me the difference is the backlog SHOULD be done, the only constraint is time, but someday/maybe is more of an open thing with some more discretion involved as to whether you NEED to do it, or ever WILL do it.
And also, priorities and GTD are still a little fuzzy for me. I get that preassigned priorities are not recommended, instead we should do constant, on-the-fly prioritization. But I don't think DA is saying that priorities are evil, right? They ARE a fact of life, right?