Actually, a couple weeks ago I asked a police officer about this. His response, paraphrased, was "As far as I know, there's nothing wrong with that. I wouldn't pull anyone over or write a ticket for it. I've seen some ricers (yes, he actually said ricers) take 'em off for weight savings. It wouldn't be the craziest thing I've seen.."
If you are really worried about it, just check your local laws and you'll be certain either way. After my conversation w/ the officer, It happened to be a bright, sunny day so my friend and I pulled my hood off and I drive around just for fun. got plenty of looks! Haha.
Personally, I wouldn't leave it off for more than a day. Too much risk with rain, debris, and then there's the risk of someone tampering/stealing stuff right out of your engine bay! Hoods are there for a reason.