My son was diagnosed with PDD-NOS ten years ago. I think it's basically an "umbrella term" for those people who have some, but not all, of the characteristics of autism. You may have good eye contact, for instance, but still have problems with social interaction, or problems with taking things literally, etc.
I was under the impression that lots of kids got the PDD-NOS diagnosis when they were ages 3-6 or so. I think that an Asperger's diagnosis is usually applied later on, maybe when a kid reaches pre-teen years. Asperger's is usually a diagnosis for those people who are able to function in society academically (may be mainstreamed totally in school), but this diagnosis has a really pervasively rough time with social skills. My impression is that a diagnosis of Asperger's usually includes really obsessive interests, but that could be up for discussion...
My son has more of the PDD-NOS/High-functioning Autism diagnosis. It's been an interesting ride, in that he seems to do better in some situations than others. He's in high school, and seems to be doing better there socially than he does at church. He's found his social "niche" in high school, and surprisingly, it's NOT with the nerds. I'm amazed by this. He's with a group of guys and girls, many who are quiet, but a few who are not. They tend to be religious kids, but not necessarily all are that way. Just seems to be a nice group. Right now, church is a struggle for him, and we are not pushing church attendance as much as we used to.