I think there may be a limit to the amount of productive work you can get done in a day, and whether you hit that limit after 3 hours or 14 hours, it's still there. So there may be some truth to your impression that news.YC isn't really hurting your real productivity.

OTOH, I've found that going out and getting some exercise, or turning off the computer and reading a book, or watching a movie tends to clear my head a lot quicker than social bookmarking sites or blogs. I feel like I get "stuck" in a low level of activity when I read too much, like all that incoming information is still taking up cycles that would otherwise be used for my startup. So maybe it's not the best procrastination activity. ;-)

I certainly have noticed that the biggest bottleneck for my productivity is the time it takes for my brain to adjust to and assimilate new architectural decisions. And letting my brain relax and switch off seems to make that go faster. (So does getting lots of sleep...again, sleep is not something to shortchange.) I'm just not certain that news.YC or other programming-related websites really let my brain switch off all that well.