As far as playing HD movies, I'm not sure why you would want HD on such a small screen but it should be able to take it (I never experienced it, but I play regular movies on it and it never gives me trouble).
For the downloading and playing music and surfing the web simultaneously, it shouldn't be a problem if you are browsing text based sites. If there's lots of flashy stuff and interactive elements, then you may experience some slowdowns, especially if you are using windows. Consider getting the 2GB ram if you do multi-task a lot, it will help with the speed since there is no swap on the Ubuntu version at least.
As far as the original poster mentioning cameras, I wouldn't feel this is an adequate machine for video editing. If you really need to do photo editing on the road I guess it would suffice but I would tend to suggest waiting until you get home or getting a real notebook if that will be your main usage. It's plenty powerful to transfer images from the camera and post them online with a bit of editing (red eyes, contrast, etc) but I wouldn't consider full scale editing.