Sorry, that is mis-information about the ISO with an XSI. Might be an issue with the XS or the XTI, but not with the XSI.
Right behind this wheel is a button that can be pressed down at any time called.... ISO. This brings up a menu (either on the LCD or in the Viewfinder) to allow you to change the ISO as needed, on the fly, without going through menu functions. I can hit the button and then use the shutter wheel to change the ISO to what I need it to be, without taking my eye off the view finder
Same for white balance. 1 button on the camera that opens the white balance selection screen. The main difference is that I am stuck with presets on my XSI, so I can't chose a custom white balance on the fly
On my XSI, when in Aperture Priority mode, the wheel located right behind the shutter button sets the aperture. When in Shutter Priority, it sets the shutter. No need to go in the menu to change either. When in Manual, the wheel works the Shutter and holding down 1 button + using the wheel works the aperture. Its really not big deal, the buttons are easily accessible. Maybe because I haven't used a higher end camera, but I don't find moving 2 fingers instead of 1 impeeds my photography progress. I actually wonder when I'd be using 2 fingers to set 2 functions at the same time?!?