Well, I wouldn't have both of them running at the same time.. hehe.
It just seems the logical place to put the thing. Here's my setup: I have a small-ish TV stand. It has only one shelf inside of it. On the bottom is my receiver, on the shelf is my DVD player. The sides of the TV stand are taken up by storage space for DVDs, where I have most of my TV seasons on DVD (to free up space on my actual DVD rack). Those shelves are too small to put anything else on. The top of the stand contains the TV itself (47" LCD) and the center speaker.
To the right of the stand is the subwoofer. To the left of the stand is the Xbox360, sitting horizontal on top of it's own box. On the outside of these two items are two floor speakers that are unused, but they act as "stands" for the two front satellite speakers.
On the inside of these two items, directly next to the TV stand, there's a small space that could theoretically fit a PS3 standing vertical. I don't want to put it right next to the subwoofer due to the vibrations that it generates. Which leaves putting it vertical next to the Xbox's box. But, I'm not sure if that's the best place, since it's low to the ground, with one side on carpet, and may not provide the best ventilation. For the same reason, I can't move the Xbox to the floor vertically because it would block the ventilation on the side of the unit. Hence the idea of putting it on top of the Xbox.
The only other place to put it would be inside the TV stand where the DVD player is, but that would severely limit it's ventilation, since the stand is open only on the front (I didn't install the glass door).
Also, the PS3 must be within a few feet of the Xbox for the optical cable connection. I have only one optical input on my receiver, and I bought a splitter so I can use both the Xbox and PS3 with optical out. But two of my three optical cables are only three feet long, and the six-foot cable is almost certainly going to be the one leading to the receiver, because of it's location in the center of the setup.
I'll try to upload a picture of my setup if I can get my camera to take anything but a crappy photo. It's old.
Thanks for the link, G.T.