This is a very good question and I think it is very controversial. My son has been diagnosed with NLD but does not have aspergers.
Some good material on the subject is found on NLDline.com. It has fantastic articles by some of the leading researchers and clinicians for NLD.
My understand, and my doctors, is that half of all children with aspergers also have NLD, but, most of those with NLD do not have aspergers.
Both children with aspergers and those with NLD share the same inability to read non verbal cues such as body language and facial expressions. They also have some of the same difficult social interactions because of this. However, NLD also comes with a host of other neurologically based learning disabilities such as problems with fine or or gross motor skills, difficulty finding themselves in space(like not being able to find their way home, for example) not being able to understand abstract mathamatical or language based learning...being that everything must be put in it's most concrete form to be understood.
People with Aspergers do not necessarily have the LD piece that exists with NLD....they can often excel in advanced science and mathamatical concepts whereas someone with NLD may only achieve proficiency up to a 5th grade level as math and science concepts become abstract.
Also people with NLD do not have the rigid interests and rigid thinking that people with Aspergers can have. My son can speak about details too much but is not overly focussed on an interest to the exclusion of others. He just cannot tell when he is "boring" the listener so just keeps going....
To make a long story short...they feel different although they also feel similar..... Their issues overlap in the area of difficulty in reading non verbal cues.
There is some speculation that NLD is part of the "spectrum" however there is just as much adamant research that says that absolutely, it is not part of the autism spectrum, it is a neurologically based learning disability, that effects the opposite part of the brain as dyslexia, like dyslexia for facial expressions and body language (this metaphor came from Sue Thompson, a foremost writer on NLD)...
I hope that his helps. Because of my son's diagnosis, i've done a lot of reading on the subject lately, said with not one hint of irony!