As far as I am aware, port scanning is legal, so you can test it wherever you like. However, port scanning in high numbers may turn the head of your ISP, and it's possible they could refuse you service because of that. The reason for this, is because depending on the severity of the scan, some sites may ban the IP you scanned from, and your ISP does not the IP's they give out to customers to be banned from certain services.
I don't care how ‘secure’ your systems are. If you have stupid people running them and using them, you can't win.
For those that know