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
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.
The ultimate prank would include a combination of that, and the one explained in this post.