I never really thought about it before this thread, but the online multiplayer of Halo and CoD are certainly the arcade/casual experience dressed up in different clothes.

I will also add that casual gamers tend to have other things to do with their lives than play games.  Hardcore gamers having gaming a major focus of their lives.

Something I forgot to add. Thinks to the industrys new "casual" meaning I went from being a gamer  to a retard seemingly overnight.

Those who say 'casual' games do not require skill, you are wrong. However, if you say casual games are easy to pick up and play, you would be right. They are easy to pick up and play (read as 'have fun'). Games should firstly be about having fun/enjoying one's self. 

A person picking up Wii Sports for the first time isn't going to get an 'out of the park ' right away. Therefore it takes a time investment and practice to get good at the game. An extra level is added when your friend starts to pitch at you, then its actually pretty hard. 
Wii Fit takes a lot of skill! Some of those balance games really make you focus to get the ball in the hole etc. 

Here is where casual games (good ones) and core games (good ones) are the same:
They are fun
They are challenging
They have replayability

Here is where they are different:
Casual games can be enjoyed without a large learning curve
Casual games are better for short play sessions (Oh I have 30 minutes before work)
Casual games are better enjoyed multiplayer with people in the same room as you
Casual games = Gameplay > Graphics

Core games have complex control schemes
Core games try to be as visually impressive as possible
Core games revolve around traditional genres such as RPG, FPS, RTS, etc

I'll also add this post from another thread that I just saw: http://gamrconnect.vgchartz.com/post.php?id=3537993

 I disagree Casual = gameplay> graphics since a lot of core PC games has very deep gameplay but is lacking in the graphics.  Until the Wii, Nintendo games system was not weak in graphics compared to other consoles. DKC had top of the line graphics in it's day.

  Ex: Civ Rev had better graphics yet more simple  gameplay  for a casual gamers while Civ 4 had better deeper gameplay for more hardcore Civ fans. Iracing & Rfactor are very hardcore  racing sims yet their graphics looks very dated compared to FM3, Dirt 2 and GT5.

I need more covertly overt lies in my OP!

Are you kidding me? Modern Warfare and Grand Theft Auto is pretty much the definition of "casual"

Hell FFXIII is the most "casual" ff game I have ever played.

That video posted above is great. Yeah, I agree. Arcade and casual are very similar. (The same?)

Arcade and casual are clearly linked. There is some grey area between casual and hardcore. Codemaster is trying to hit this grey area with F1 2010; Not too casual yet not too hardcore  either.   It still will not be an arcade racer like Mk or NFS.

Red Dead Redemption is IMO also in this area, most of the gameplay was simple and easy  that I felt a 8 year old could easily finish it yet it's not really an arcade nor as casual as Wii Sport.

 PC leans more toward hardcore while consoles leans more to arcade/casual even with the same game. This was true even back in the days of  NES.  Silent Service was extremely easy (dumb down) on NES compared to the PC version. Airplanes can carry 30 plus bombs and 50 plus missiles  on consoles flight games.

Think about the business model coin-op arcade games followed.  A coin-op machine lived or died based upon getting people to put money into them and keep playing them.  They had to resonate quickly and get you playing, and then ramp up the challenge to keep you coming back.  They had to be accessible and also deep.  Nolan Bushnell happened to discuss this also in an interview, explaining about the art of this.  So, there is an art to this.  Also, look at the modern coin-op machines and see what they carry.  I see Megatouch machines, and I see what is seen as "casual" stuff.  Golden Tee and hunting fit into this also.  I would also suggest to look into iPhone stuff, or Flash games people play.  They are simple designs you play quick.  Usually the games are time killers you play for high score.

Now, what has happened is that the artform of videogames has been expanded to cover a wider range of play experiences.  The gameplay got deeper.  Has it gotten more challenging?  I would question if that is so.  Storytelling is the hook now, not challenge.

By the way, the connection between arcade and casual is important to me.  Because of Game Room on XBox Live coming about, I got into casual and retro gaming with the CADERS group.  It is up on facebook.  I do see connections here.  But, like this thread shows, what is exactly "casual" is tricky to nail down.