Matt Laur , Aug 10, 2010; 01:29 p.m.
Unless you're also able to specify and control how the images from these different devices are all processed before you can see them, such a test doesn't help, really. Post production (or even just in-camera JPG rendering) will have as much more to do with the perceived differences than anything else.
Of course, if you're doing your test on low-light subjects, and thus are using higher ISOs, that's going to produce very different results as well. And, you've got to be very careful when describing "300mm equivalent" on different bodies. What do you mean by that? Are you talking about keeping the perspective the same, including the background and foreground elements? And, are you talking about ignoring, or trying match up the inevitable (and significant) apparent differences in depth of field that each format will render, under some circumstances?
This is a lot more complex than just "take the same shot three times with different camera bodies."