Is Hacking Legal?

Probing
In 1998, the Norwegian Supreme Court ruled that scanning/probing a system is not by itself illegal. The ruling is based upon much the same legal and technical basis that you might find in other countries. Right now in the United States, there are few laws on the books that regulate such things. Not only are the laws deficient, there are some technology problems as well. The biggest hurdle is IP spoofing, which would allow a hacker to send scans at your system while pretending to be somebody else (it isn't useful to the hacker other than getting somebody else in trouble). In summary, chances are small that you could prosecute a hacker for scanning your system. 

Break-in 
If a hacker successfully breaks into your computer, you could go after the hacker. However, the hacker can come after your machine from anywhere in the world; most countries have laws against hacking, but trying to prosecute them from far away is very difficult. The police will not want to get involved unless there is a major financial loss involved. In order to get them involved, it helps to have airtight evidence. Getting such evidence can be very difficult. One way to start the process is to have a complete packet log of everything the hacker did. In addition, you will need to personally harass the ISP where the hacker came from in order to find out who had that IP address during that time. 

Other resources
The new legal force: The USA Patriot Act of 2001 can be found here.
The antiterrorism act will significantly hinder the open disclosure movement and could result in a lifelong prison term for those that deface networks and the attempt to sell exploits. This law will have serious impact on both sides of the security field and what they're trying to do.

The Federal Bureau of Investigation has listed recently prosecuted computer cases online here.
The Electronic Frontier Foundation has compiled information here.
The US Department of Justice has compiled information here.
The US Department of Justice has compiled additional Computer Crime information here.
The University of Dayton School of Law has compiled Federal information here.
The University of Dayton School of Law has compiled state information here.