You are allowed to bring a laptop or netbook into the parks.
As far as which way to go I have a couple of suggestions. First, make sure you know the advantages and limitations of a netbook. A netbook is designed around using offsite processing or cloud computing, like using Google Docs instead of Office. If you are planning on doing anything processor intensive like photo editing or running multiple programs you might find the ATOM processor lacking.
I might also suggest paying a little more for a solid state drive. They are a bit more epensive than a standard hard drive and will be smaller capacity but there are advantages. They boot quicker, run cooler, use less power which is nice when you are running on batteries, and they have no moving parts. If you are going to be running the machine while you are in motion on the chair the no moving parts is nice because the most common cause of hard drive failure in portable devices is from jostling.
If you are planning on getting an XP based netbook instead of a Linux based one I would also look into upgrading to Windows 7 when it comes out. The beta version runs great on netbooks and you will get all the added benefits of the new architecture that you lose when using XP.