Does anyone have a good starting point for learning about development for the new Nintendo DSi? What kind of hardware do you need to develop and test something like this? Can you develop for the DSi using the standard DS? What language do you need to use?
[Bonus Question] : I also heard the new DSi is going to get an App Store similar to Apple - Are developers going to be able to sell games using this?
As far as I know, the only legitimate way to get the Nintendo DS/DSi development kit is to get it straight from Nintendo--requiring you to have a lot of money and preferably a signed contract with a large game publisher. This is the case with all of the major game consoles, with the possible exception of the Xbox 360 and the XNA framework. I don't foresee them changing their business model soon; controlling the SDK ensures that the hardware manufacturers will get their cut of the profits for any game that's released for their console.
If you'd like to get an idea of what DS programming is like, there are some "unofficial" (read: hacked and breaking license agreements) homebrew DS programming environments. Searching Google for "DS Homebrew" will probably turn up a lot of information.
For what it's worth, most development for game consoles, the DS included, is done in C or C++.