DSi has two cameras, and the SD card slot. No GBA slot.
I could do that with a DS, just give me a camera phone and then a few weeks to make one.
As other sites have said, the cameras are more or less a gimmick (POINT THREE MEGAPIXELS - 300 000px.)
Although with it now, it now actually feels like a PSP but smaller single screen and 2 cameras. From the ad, it's possible to run .nds from the player, but since there's no way to physically connect the DSi to a computer as we know of so far, then no way to change things around PSP style.
I'd stick to my DSl. If you don't have a cameraphone, and no DS, a DSi is an easy way to just get it out of the way. Now we need a viable phone/voice communicator for the DS.
To think of it, one could make a mobile phone using the DS, however you'd need to modify the GBA slot filler. No idea why they supplied a circuit board with it.