Would it be considered cybersquatting if, for example, someone registered the domain lexforums.com when an already established website was running under the domain lexforum.com?
Well, if Lexforum is viewed as a trademark, then it would fall under cybersquatting. I'm not too well versed in trademark/domain issues though, but from what you want to do it seems as if you want to profit of lexforum.com 's traffic and name. It's possible that they could prove that your registration was in "bad faith" and force you to hand over the domain to them.
Even if it is just an example, the principle remains the same. By registering it in that way you are apparently trying to gain from the original sites' reputation and traffic which could be seen as a "bad faith" registration.
Domain name investment (Domaining) and cybersquatting is different. A wiki article might also help you a bit in understanding it. http://en.wikipedia.org/wiki/Cyber_squatting
I have a great cyber squatting story. Wakeboardingmag.com holds an award show for the past like 5 years or so. They call it Wake Awards well I bought Wakeawards.com and Tried to sell an email to the Senior Editor of the Magazine asking if he wanted to buy it. He wasn't to happy. A couple months later I gained up the courage to just give the company the domain. Once I did this he gave me tickets to the award show for as long as its running, gave me credit for the domain on the site they are producing for it, a college reference and a Free life time subscription to the magazine. S
Of course they'd have to prove your registration was in bad faith. If your site and content clearly have nothing to do with the original i think you're fine