It wouldn't shock me if the DSi was actually a great deal more powerful than the DS lite. After a year or two, Nintendo could unlock that extra power for developers. By then, developers will have started taking more advantage of the DSi's extra features anyway, like DLC and the cameras, making the DS lite and phat more and more obsolete. Then Nintendo can flip the switch and the DSi makes a clean transition to pretty much being "DS 2".
Of course I'm just speculating, but it would not surprise if something like that was the case.