The hardware comes preinstalled with additional functionality. This additional functionality means the DSi can potentially have games that it can only play. Unlike and Xbox with a Vision camera you cannot purchase the addon seperately and make a DSi out of a DS. This makes them different, whether they are different enough is subjective. I fully expect the console manufacturers to do similar updates to the systems which effectively preclude previous models.

Nintendo themselves have said that it is not a new system, but just an upgrade like the DS Lite.

The DSi can play (most) DS games - but the DS cannot play DSi games.

If DSi only games came to retail, then the issue would get more murky - and they would, in effect be two separate consoles.

The DSi has a *lot* more RAM than the DS does, and runs at double the clock speed. The screens are the same resolution.

I'm not sure about graphics capabilities - if the DSi was capable out outputting double the polys that the DS could - that would be cool.

Updates:

Bigger Screens (not a HUGE difference, only minimal, didn't need it IMO),

Camera (crap quality, really nothing fancy),

DSiWare (will be nice addition, but not a need),

Sim Card support (it's ok, but nothing amazing, should have kept the GBA slot and just added a new slot IMO).

Overall, if you have a DS Lite, might as well not even upgrade. The only real features here are DSiWare and Sim Card support.

That's my opinion of course.

I'm looking around for Viper1.

I know what he's gonna say - the DSi and the DS are two different consoles. There's an objective standard for this sort of distinction, and I can't remember exactly what h said last time but I remember the reasoning covering the Gameboy Color and the DSi both.

That said, Nintendo is probably never going to report the DSi separately, because in the eye of the consumer it's not going to be any more different from the Lite as the Lite was from the Phat.

It's basicly the gameboy generation all over again (http://www.vgchartz.com/forum/thread.php?id=65421).
Appart from the ovbious paralelism between DS=GB and DSi=GBC if you take software into consideration. There is also another one, Firmware. The jump from DS to DSi is similar to that one that Nintendo did from Gamecube to Wii.

So yes, DSi is a different from DS, but totally backwards compatible.