The 40D, 50D, 7D bodies and up... they all have 2 wheels for controlling your settings. The Rebel series like the Xsi or the T1i all just have one wheel. So you have to set a setting, push a button, then twist again to set the next setting.
With my 50D, I can twist two different wheels at once, which is very easy to pick up, and basically adjust to new situations just that much faster.