Maybe it's just me, but this seems like a great prank if you're moving offices at your job or if you're leaving...it's not doing any real damage, but it's certainly leaving a mark. Of course, if they aren't too computer savvy and don't have an IT department, this might affect your future reference when they claim you trashed a computer before you left.
Link Via Geeks Are Sexy
The ultimate prank would include a combination of that, and the one explained in this post.
127.0.0.1 facebook.com
127.0.0.1 www.facebook.com
facebook will not load on this computer. rinse and repeat for your friends favorite sites.