to make a parallel with a house is a good idea because ur computer is ur property and contain private info just like ur house...
It's illegal to use hacking techniques to penetrate an unknown system. Most times when security flaws are found it happens in one of two ways:
1. A smart guy sets up his computer and tests it for security flaws with another of his computers. He owns both machines and thus the law is intact.
2. A pro is hired by firm to do a security check-up, he finds some new and voila he gets a bonus... a professional hacker...

I would have no right what so ever to e.g go and see if someone's front door was left open. I do however havce the right to leave my own frontdoor open and use my own house to make a point or test something. In court there often isn't much doubt about computer crimi. If you penetrate a system you don't the right to be accessing then you break the law, where the big troubles start is when the judge has to figure out how big a sentence the computer criminal should have. And we all know that 5 years behind bars for defacing a website is insane but it happens.
Many of you AO users might think that trying to access someone's 'puter isn't illegal because, what's the big deal, right? And often it isn't a big deal but I can be and that's why it's illegal. So to hack legally simply make your breakin attempt the same way that you might make an physics experiment. that's the only professional way to do it, and oh yea, probably also the only 100% legal way