nrapello has made a great summary.
adding homebrew software to a DS isn't really "hacking" per se - it's more or less putting a card in (just as you would any commercial game) and running an application like you would from windows or linux or OSX.
Not totally sure about the effect on your warranty in other countries but here (Australia) it's perfectly legal, in fact, here all kinds of modification chips for game consoles are legal.
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