Cave Story designer Daisuke "Pixel" Amaya told us in an interview last March, he's been working on a version of his vertical scrolling shmup Guxt for the 3DS, and Nicalis, who's publishing the game, has just tweeted recently that it's been rated by the ESRB and is ready to go. You can download and play the game online for free on the PC right now, though the game will be updated for Nintendo's handheld for sure.
One of the game's producers has tweeted that the title will have a "vertical display mode", so players will be able to tilt the 3DS to hold it vertically, and see the game on a correctly-aligned screen, rather than having a bar running through the middle of the action. Unfortunately, there's no word on exactly when the game will arrive, but considering it's been in development for a while., "soon" is probably the answer you're looking for. Stay tuned.