that's great advice in spirit, but terrible advice in reality. At the least, your ISP might ban you, and at the worst, the cops or FBI could show up to ask what you are doing.
There are a lot of thoughts on this subject over on the nmap.org site on messing around with data on someone else's computer, be that a private or public service.
On a related subject. I accidentally mis-configured remote desktop on Ubuntu, I selected the 'configure network to automatically accept connections' which uses uPnP to open a port on your router (poorly named I think).
Someone ran a port scan on my IP and noticed port 5900 was open and decided to connect to it, I had my computer configured to automatically accept connections (because I use the iPhone VNC client as a remote control), I was quite glad I was using my computer at the time and noticed them connecting!
My first reaction was to run NMAP against there IP, I guess thats probably a bad move!
http://nmap.org/book/legal-issues.html
There are a lot of thoughts on this subject over on the nmap.org site on messing around with data on someone else's computer, be that a private or public service.