Any programmer which ever developed something on a console (PC programmer move to another page... kidding ;-P) know that each and every generation and brand offer very different physical architectures. The Saturn and Jaguar may have been killed by their complex architectures though I think they were ahead of their time.
On a console, most of the machine is dedicated to gaming and multimedia and one of the most important advantage is... the hardware won't change for a while. Though the lines are starting to blur with the actual generation of consoles, on a PC, it's another story.
When it comes to put out a game which is cross platform, how well does it goes? Will the game be offered on the trinity which is PS3-Xbox360-PC? I grouped those together because they play the same card : powerhouse. Wii, DS, PSP, iPhone....etc, are just in another area hardware-speaking and are hard to compare because the play the "my features are cooler" cards.
So. Every once in a while we see "graphics comparison" articles usually about PS3 vs XB0x360. I think it just doesn't make sense. Because translating code from one API to another, using very different architectures relying on different memory and CPU usages can be very challenging since you must come as close to providing the same visual experience. However, they don't have the same GPUs and CPUs so how well can you translate 1 to 1 every textures, renders, 3D models, particle and lightning effects, etc. Then you have the compression problem. I heard that programmers still do a lot of compression/decompression on the XBox360 because of space of the disc (still being a DVD) vs the PS3 which is on blue ray.
Attention please : compression doesn't mean lesser quallity but CAN mean alteration on some level. How well can you come out with things looking exactly the same on each machines.
The NVIDIA PS3 GPU and XBox360 ATI GPU are different in their architectures as well. Running a game on a PC with an NVIDIA vs a PC with an equivalent ATI won't render exactly the same. Because they both have strenght and weaknesses when it comes to rendering images.
So here is the challenge. To create the same visual with very different hardware and code.
I must kneel to those programmers working on the engines because I always think that the differences are very subtle. We often have to zoom in parts of the images to see it as obvious. I don't know anyone doing this while playing.
The only field where I think it can affect graphics is the brute calculating force of the CPU which calculates the physics and mechanics of the game. You may end up with fewer physical effects (flames, collisions, destroyed pieces of environment, particles etc) or fewer monsters. But those are rarely compared. We only see compared stills of the same game.
It can become quite dramatic when you take a game which targets the PS3 for example and port it to the PS2. The result can be very strange since you have to cut down many textures, monster number and even bosses (the kraken in Tomb Raider Underworld was taken out for the PS2, my guess is technical issues).
So to conclude about those graphics comparison, I think it's a good marketing tool companies can use to attract casual gamers or visual maniacs. However, it doesn't affect the gameplay at all for those standing in the middle. Even comparing those platforms with the Wii is irrelevant because the platform was not meant to be pushing it's brute power as its main feature.
I still think that graphics comparison is irrelevant. Sometimes the features of a game differ from one platform to another. This may be an argument to prefer one platform vs another.