I'm interested in purchasing a netbook or notebook PC that can run Flash HD video like Hulu, ABC.com, and HD Youtube. What are the minimum spec CPUs required to play this content without choking?
Each site uses a slightly different flash player (generally custom) though they all use similar encoding and bitrates. You will find that at the margin, some laptops will play most sites but not all due to 'extra' stuff the flash player is attempting to do beyond the audio/video.
However, online HD flash video generally consists of low bitrate h.264 encoding, and compared to compressing it, decompressing it is relatively painless, especially since they are generally encoding to about 720p 12-24 frames/second.
Any computer with a 2+GHz single core or 1.6ghz dual core that's not bogged down with any other processes (background virus scan, web browsing, etc) should handle web HD video just fine.
Going further, if you want to decode real HD (ATSC tuner, blu-ray, torrents, etc) then you really need something better than 2+GHz dual core.
You'll also find that in the lower end machines the integrated graphics and low memory will get in the way. You may need a slightly faster processor and memory bus (and more memory) to compensate if your machine doesn't have dedicated video memory.
Well... it depends. Not only on your CPU, but the operating system as well. Unfortunately, I've noticed that flash videos aren't as great in linux as they are on windows (or even OSX). So what operating system you'll be using should be taken into account (plus, at least the last time I checked, ABC.com required you to use windows =/)
That said, netbooks in general won't be good for watching HD videos (yet). I'm sure it's possible, but there will be lag.
If you get a core 2 duo, you should be fine. You can get them super cheap these days, so it shouldn't be a problem finding them. Heck, with my old 1.8ghz centrino laptop I can play HD flash, so mostly any other things these days should work.