I think defintions are really being mixed-up in this thread. Why does everybody suddenly insist on defining every single game on it's hardcore or casual-appeal?
Gamers can be defined as "casual" or "hardcore" so I think we can agree that there are games that appeal (mostly) to hardcore or casual gamers. Still I don't think that this is just a black and white story. Not every gamer can be defined as hardcore or casual. Most of us are probably somewhere in between.'
People also seem to be confusing successful games with casual games. Just because lot's of people have been playing Mario games years back does not make the franchise casual. Things are just not that easy.
There are many explanations: Mario was very successful, heavily marketed, accessible... (and I also disagree that being accessible is enough to qualify a game as casual...). Modern Warfare 2 is extremely successful yet I doubt it is a casual game appealing to people which have not been gamers traditionally.
For me casual gamers have (recently) (re)entered the video game market with intuitive products such as Wii or DS and online games in the recent years. They don't spent as much money or time on video games as "hardcore" or active gamers do.
From this point of view, NSMB Wii appeals to both groups: it is a title you can easily pick up and play with your family or on your own. It is also a title which appeals to long standing fans of the franchise.
Anyways... like I said before... people, especially editors and analysts seem to feel the necessity to classify every single game as either hardcore or casual and more specifically tend to label 98% of all Wii games as casual without really thinking about who is actually playing the game.
Oh and btw... I think people calling GTA a casual game is a great example of the general confusion about labelling games as casual or hardcore. I am not saying that I have the one correct answer to the debate. But from my point of view GTA is clearly a game designed for and appealing to hardcore gamers as I can't picture moms, grandpa's or some other folks enjoying Dr. Kawashima's brain training picking up GTA and blasting their way through liberty city.