How about having your hostfile swap script change your IM status to something like "Goofing off online." (or whatever)? Nothing like the shame of broadcasting that to your coworkers to get you to keep your goof off time to a minimum.

This is a great idea. Mind sharing your script? I block annoying sites with my hostfile, too, but every once in while I actually need to look at something on Facebook. This would also be useful as it's a big mess with all the changes I have to make for work, too. Storing them separately would be super helpful.