I'd also add that the longer you wait, naturally computers get faster, better, and cheaper.
If you intend on playing anything that calls itself HD, I would get the latest netbook with the best video specs you can get (screen resolution, HDMI output, and generally advertising that states it will play HD content).
As Thisisnot pointed out, you will likely need to gather the specifics of the shows/movies you intend on playing so you can compare those specs to the machine(s) you are looking at.
Unless I'm wrong, you're not going to be able to play BluRay quality on any netbook today. DVD quality, definitely. Then there's that fuzzy line between DVD and BR that everyone loves to call "HD".
-Eric