Eh, a while ago there was a bricker going around, but I've never heard anything about it being implanted into commercial ROMs. Running it in no$gba probably wouldn't do anything, but if you use something like OfflineList to identify your ROMs, you'll know for sure that they haven't been modified in any way. As for homebrew, just don't download from anywhere you don't trust. If you're still nervous about it, you can install FlashMe, a hacked firmware which includes recovery code in case of your DS getting bricked. If your DS is fairly recent though (say, in the last year, maybe two? Anyone have better figures?) it probably has its firmware protected in hardware. That's assuming your DS didn't sit around in a store/factory or be second-hand, obviously.
