I'm curious about working in the NICU also but I'm not sure how you get into it. I think it's something to ask about when you do your affiliations, to see if you can do some of your training in neonatal. I think it's a somewhat difficult field to break into unless you get some exposure to it while in PT school.
My curriculum at Emory University in Atlanta has a lot of emphasis on pediatrics and we learned a lot about babies (too much for most of us!). It's definitely a niche field which is hard to get into, but not impossible.
Generally, you have to have a lot of experience in peds to get into the NICU (clincals, cont ed). However, there aren't many NICU's out there so the jobs are most likely slim. I know at CHONY, they have PTs specifically for the NICU, but in other hospitals, it's not as extensive. Most of these hospitals have some type of training program for it's staff.
Before PT school, I worked in a children's hospital as an aide and the scope of what a PT can do seems to be limited in a NICU (but still rewarding). It is a very specialized portion of peds.
Below is info of key elements to work as a PT in the NICU: