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.
I did this when writing my dissertation. It worked really well. I find that most of my browsing comes from clicking through to the sites in my "Most visited sites" homepage. Blocking them was a good neg that broke this harmful browsing pattern.
It's the exact same thought process that, after I've been up all night and I'm trying to stay up all day to reset my internal clock, convinces me that it's ok to lay down in my bed for a minute. I inevitably wake up quite confused a number of hours later.
I saw a site once that you can route your procrastination sites to. When you browse the site (by IP address, naturally) it displays a message saying basically "you blocked this for a reason... slacker." Since I can't remember the address anymore, I've been thinking about installing a web server on my laptop to emulate it.
Edit: I found it. The address is 188.8.131.52 and it just displays the message "You've chosen to block this site in your hosts file."
For me, it's a subconscious thing. It's close to mindlessly browsing.
Forwarding to that message wouldn't do anything for me. I think the most important thing was just that "neg" to break out of the mindlessness and get back on track :)
Sure you can overcome it using various ways, but for me the fact that it gets in the way is enough to prompt me to go back to what I should be doing (i.e. something productive)