For example, if I were to put sites like reddit or HN in my hosts file, and point them to 127.0.0.1, it really wouldn't take me more than 30 seconds or so to get around it. It should work as a reminder that, "oh yeah, I should be doing something else" but that never seems to work for me.
In this case, I have to assume that just logging out of HN is enough to get around the noprocrast. Assuming also that when the noprocrast is in effect you can't access HN to log out, it still only takes 30 seconds to kill the cookie(s). Even with some sort of unkillable cookie, I do webdev, so I've got easy access to Firefox, Chrome, Opera, Safari, and IE8. Even with something like a flash cookie which (I think) remembers you cross-browser, I also have VMs to run IE6 and IE7. Admittedly, by the time I boot up a new VM to visit HN, I'd definitely get the point that "oh, I should be working" but that's a bit beside my point.
Besides, HN is nothing compared to the time sink that is StackOverflow. There's always something I can research ad nauseum there. I've learned a lot from my procrastination, but I've got shockingly little done.