As Alex said, PDD-NOS won't be a new diagnosis soon; you'll retain it if you already have it, though, just like Asperger's.
That said, PDD-NOS is all over the spectrum. It's not definably "more" or "less" severe than anything - the point is that it doesn't fit into the spectrum that way. That's why it's "NOS" - "not otherwise specified."
Basically, it just means you don't fit any defined criterion on the scale. That could mean that you have almost all of the symptoms of autistic disorder, but are missing just one "required" symptom. It could mean that you clearly have some kind of spectrum tendencies, but you don't have enough total symptoms to qualify for any of the other diagnoses, Asperger's included. It could mean that you fit half of the Asperger's criteria perfectly, but don't have the other half. It could mean anything - more severe, less severe, in between, or almost exactly the same (with a tiny exception or two).
In general, in North America, it's used to describe something less severe than Asperger's, whereas in the UK, it's generally used the way you describe (something in between Asperger's and autistic disorder). I must stress, however, that these are not standards for "what PDD-NOS means," it's just how they're most commonly used in each place. PDD-NOS can, as I said above, mean anything with regard to "spectrum status."