I feel you should get permission from the owner before you do any exploring of someone's system. If they want a security analysis of their system, they should be the one to initiate it. It should be a voluntary process. What would happen if you went through the parking lot at Wal-Mart trying to open people's car doors? When the police drive up and say "What are you doing? Come down to the station with us," what would you say? "I'm just checking people's security; I wasn't going to take anything!" Would the police accept that argument? On occasions when I have had trojan placement attempts on my computer I have port scanned the attackers and found what I thought were trojans on their system. I have called or e-mailed this person's ISP or network to let them know one of their clients or machines might be compromised as a zombie. To me that's ok. On the other hand, if I found running trojans on their box and used them to go inside to explore, I would be in the wrong. There's a fine line, but once you cross it, you're in the wrong, IMO. :)
