I cant understand why people upgrade from rebel A to rebel B anyway. They're supposed to be an step into the world of DSLR photography. You make ur experieces with the Rebel and one day when u learned enough its time to step up a level by going Full Frame or at least choose the best APS-C there is in that time.
But going from beginner cam to....another..beginner cam? seriously? There must be _really_ good reasons for that. Makes a lot more sense to buy good lenses instead of updating the rebel cam. Or just save for 5d mk2/3
I recently upgraded from 350D to 60D. Could just as well have upgraded from 350D to 600D really, sensor-wise at least (exact same sensor in 600D, 60D and 7D). The only thing the 60D offers on top of the 600D is some weathersealing (which you might not need), higher burst rate and more cross-type AF points. If I wouldn't want to do sports photography, the 600D would have been plenty. Upgrading to it from the 350D makes a lot of sense. 2,5x resolution increase, 2 stops higher ISO capability (ISO expansion not included), video functionality (350D had
none), larger (swivel-type) LCD screen, flash master functionality...
If you're coming from something like the 350D and sports isn't your priority, then the first
significant upgrade beyond the 600D is the 5D mk2. But its price is about 3x as high. So if you're budget-limited, upgrading from a rebel to another can be a very worthwhile decision. Rebels can be very far apart, especially with the introduction of video.