It seems as though the first 3DS flashcart has been announced by Gateway. As seen in the demo video, you can only load one rom at a time, but just for now?
It is in the final stages of development, but it already claims to support all current roms and 3DS and XL. If we believe the video, one of the things they need to do is to create firmware for their card. So we don't have too switch SD-cards.
Ah! *rubs his hands* News! If this is real then creating the Firmware should be a piece of cake. Maybe I think again about buying an japanese 3DS to play all the games that aren't released here.
Without a firmware to actually load different games, I'm not sure this will be all that amazing. The whole joy of a flash card is tons of game in one card. And wihtout a firmware I seriously doubt you can play games of other regions, which is also one of the main reasons to buy a flash cart. This device is 100% just for piracy. I can't even try to play itself off as a product that has any proper uses. I'd personally recommend you update to whatever firmware is current once this releases, then do not do any more updates after that and just wait to see of people come out with cheaper clones or better carts that actually run firmware. Keep in mind, without firmware, Nintendo can just block it with a update and it will be blocked forever.