Basically all the parks are open and have countdown celebrations for NYE. The parks tend to fill to capacity with disney resort guest be the last to enter. If you are staying offsite, once the parking lot is full no more offsite guest can enter the park. Mk is usually filled by 10am and the other three parks follow within an hour. Best to get there early and stay. Call now for PS for NYE because if you wait your choices will be limited to counter service locations.
PI & Cal Grill have special NYE parties. Here is a site with some of the activities from last year, will be pretty much the same. She will update it as soon as she hears about this years festivities.
http://wdwig.com/nyeve.htm
Several discussions regarding this holiday on the previous pages that might help you.