Asteos: portscanning can be considered illegal.
In Moulton v. VC3, a federal court found that the costs incurred investigating a port scan did not constitute damages under the federal Computer Fraud and Abuse Act. Moreover, the court found that a party's port scan did not access the other party's network. Port scans elicit information from computers and computer networks; under many of the state statutory definitions above, port scans do access computers and probably would constitute computer crime. Certainly, the court could have found port scans to constitute access under the CFAA and found the party liable for the port scan.
Most state computer laws prohibit unauthorized connections to other machines. Which you can interpret however you like, depending on how good you lawyer is. The previous qoute states that the loss of bandwidth is so insignificant that it wouldnt constitute enough loss to be considered damages, but it is basically trespassing which is illegal. It's also most likely against the terms of servce of your ISP. Since they write the laws so loosley they can be interpretted however prosecuters would like to.
Here are the U.S. Laws concerning computer crime: http://www.usdoj.gov/criminal/cybercrime/1030_anal.html
They have actually reformed it so that something as negligible as unauthorized port scanning is no longer a crime, but this is federal law not state.
If you dont live in the U.S. and think port scanning is legal check out this: http://www.mosstingrett.no/info/legal.html