By hacking I am not referring to an illegal activity (that is cracking).
The phrase you're looking for to use in Google Search is "penetration testing" as that is the generally accepted description of what Ethical Hackers do within their own organizations, or for other organizations such as IT security consultants.
There aren't a lot of wildly popular general discussion forums about the topic, since it really is a nichce area of IT. People who do this legitimately take it very seriously. Some places to get you started in finding good discussions:
Paul Assadoorian of PaulDotCom has a podcast and forums on his site. Some communities using certain open source infosec tools might have IRC channels and/or mailing lists. Check out the NMAP, Metasploit, SNORT projects for starters.
The SANS Institute has a mailing list for alumni of their courses that discuss a large number of security topics including penetration testing. Of course you have to have taken a SANS course, but if you're serious about this topic, you might want to do that anyway.
Not exactly a forum, but Reddit's netsec section has very good articles/discussions related to network security.
I contend that 2600 magazine is the best hacking forum there ever was.
Backtrack is a linux distro designed for penetration testing with a rather knowledgeable community built around it.
Just make sure to Google and search the forums before asking a question.