I voted that it's not cheating. It takes two to tango, and as the third party you're not the cheater. Although often it's still a bad idea, for many of the reasons already mentioned.
However i'd like to present a experience with a slightly different angle; what if the cheating works out for the best?
I knew a couple, they had been together for years, were living together, but they had grown apart, it couldn't really be called a relationship anymore, they just weren't admitting it or doing anything about it (even though everybody around could already see that their relationship was pretty much non-existent).
Then there was a party, the girl was there, I was there, things happened. And this caused the girl to finally realise and admit the relationship was already over, and that she didn't want to continue with her boyfriend, they simply had grown too much apart. She broke it off, the BF resisted the change (denial was a big element in his behaviour both leading upto and after the breakup), but eventually accepted it.
Some nastiness ensued in the short term, but after a while things settled down. Girl and I dated for a while, didn't work out, but no nastiness there, it was fun while it lasted. Later she met another guy, much better fit for her, currently in a good relationship.
Occasionally see the guy, who is also now in a new relationship, don't know much about it, but AFAIK everything's fine there.
So although this wasn't a perfect breakup, the cheating at least forced them to admit to the fact that their relationship was over and wasn't coming back. It's guessing ofcourse, but otherwise they would have likely spent a long time still clinging on to the rut they were in.
So that's a situation where i believe the cheating actually was a good thing; a little pain but it prevented a whole lot more pain in the drawn out process that otherwise probably would have happened.