One unanticipated feature of the netbook vs. laptop for our nearly 5 year old is the keyboard size. The smaller keyboard can be frustrating for adult fingers but allows DS to type much better than he does on a full-sized keyboard. Of course, you lose other feature because of size so it is a trade-off.
As I mentioned before, we have the Classmate PC (aka 2Go PC) for DS. I have the EeePC and a laptop. I love my EeePC (for me) but much prefer the Classmate for DS. They are about the same price but the Classmate has so many more usable features for him. I am not familiar with the other suggestions listed here so I can't help with comparisons on those.
