The term "HD Twins" has been thrown out there in regards to the PS3 and 360.  While one can argue the quality of game graphics are similar, and that they both are being beaten by the Wii.  Well, as I see it, it is close to pointless to call them Twins:

1. Neither 360 or PS3 fanboys are going to ever give the other system its due.   Both will claim their system is CLEARLY superior, and to say they are alike is an insult.  Is there a PS360 coalition somewhere for whom grabbing over 50% marketshare among the two is critical?

2. Did anyone ever bother to call the PS2 and XBox and Gamecube "Tripplets"?  Just because systems are similar in specs and capabilities, doesn't make them "Twins" or "Tripplets". 

3. And how does the Wii make anything different?  Because it is different to a larger degree than is historically normal from competitors?

And let's talk a second about all these predictions:

1. How the heck does one even know what companies have coming down the pipelines that will be game changers?  Natal may or may not be it.  And who the heck can say that "hardcore" gamers will flock to either Natal or the PS3 wands?  Is it suddenly better graphics going to make the Wii approach more appealing to the "harcore"?  I hear the arguments how the Wii is embarrassing and not fun to waggle.  Now high def graphics makes the control mechanism more appealing?

2. And what exactly do you win if you are right?  Do you even get an image you can add in your profile for being right on?  Maybe it is a monkey at a typewriter?   Is there some award ceremony party you get invited to?  Or, do you get some cookies?  If you win cookies, what flavor cookies?

3. And what do you win if your console of choice comes in second?  I said SECOND, because I want to see anyone who can argue that the Wii will be surpassed with a straight face.  Straight face here also means not one that looks like you are a raving partisan lunatic.

If anything point number 1 confirms the HD twins are...twins.

Their fanbases act exactly the same.

In point 2, the PS2, Gamecube, and xbox were different from each other enough to not be called triplets. Ps3 and 360 share 90% of the same library, and 95% of high quality games. Similar control schemes, exact same target audiences, both are driving their respective networks hard, both are nearly identical graphically.

IF you want to be more specific, PS3 and 360 are not identical twins, but fraternal twins. They are both very similar, but have enough differences as to tell them apart from another. But it's easier to say "HD twins" than "HD fraternal twins"

Simple answer - HD gamers combine the two HD systems together so that they can find a way to say that they are winning against Nintendo.

The longer answer is - the oldest gaming rivalry was between Nintendo and Sega. When Sega stopped making HW, the tide went in favor of Sony, or the new Microsoft venture, the Xbox. These ex-Sega fans hated Nintendo so much for "destroying" Sega, (sega killed themselves, fyi). They began to recruit other gamers to hate Nintendo. This brought about the era of "kiddie" labeling of everything Nintendo, because apparently "mature gamers" played PS2 and later, Xbox (and only Xbox was given a pass because it had Halo). Everything about Nintendo had to be spat upon, looked down upon and laughed at. "Fisher Price" controller of the GameCube, "Waggle" and "fad" of the Wii.

When the Wii received its success, these angry people had to find a way to knock it down a few pegs, and they figured that if PS3 and 360 fans joined forces, they can say that together they outsold the Wii.

Ok, the real truth now - because most of the game libraries are similar/multiplat and the devs can double the amount of fanbase where Wii exclusives are .. well, exclusive to the Wii.

But admit it, the story of fanboy origins is a lot more interesting.

They're not even twins, they were born a year apart. You could call them HD brothers?

One is Black the other is White, born a year apart.... how is that twins?

Both have 512MB memory. Both have similar (in performance) CPUs and GPUs. Crossplat development between these two machines is arguably easier than any other two consoles in history.

The Wii is a standout, architecture-wise.  Hence its not included, and the other two get the "twins" moniker.

They are called twins because they are both substitutes of each other. They both have more or less the same value curve. And yes, the last gen could also have been called 'triplets' by the same standard.

i hate that term like poison.

One twin is always born first, and they have multiple colors

They're the two most similar consoles ever.