Well, that's not necessarly true cmoney. NOA will totally honor the warrantee of the Japanese NDS, at least thats what a NOA phone rep told me. But, if the DS has to be sent to Nintendo and can't be repaired at local Nintendo store, (dunno if these even exist these days... last time I had to deal with Nintendo for warrantee work was with my NES...) then it will have to be shipped off to Nintendo in Washington state.
And the Nintendo phone rep told me that they can't guarantee you'll get back your Navy Blue DS lite, since well Nintendo America only has the Polar White DS lite and Polar White casing.
Edit: So... yea NoA will honor the warrantee since you kinda might run into a language barrier and shipping would probably be expensive. But it does have its draw back as well.