So you might have noticed that the DS is in a bit of a drought in terms of new and awesome games. With the announcement of the 3DS it may seem that the were in a bit of a void until its future release.
But if your still in the mood to do some gaming on your way to work or on your summer vacation, I would recommend checking out Dementium II for the DS. It is the sequel to Renegade Kid's original Dementium released back in 2007.
In my opinion Renegade Kid is the definitive FPS developer for the DS. I've tried other franchises, such as Call of Duty, but so far nothing has comes close to what Renegade Kid can do and does with the DS. It's very impressive for basically just a few guys running the show.
Dementium II is more of a FPS adventure than a shooter as you play as William Redmoor. Poor Will has awoken from a brain operation, conducted on him by a crazed Psychiatric ward doctor. In the first Dementium (spoiler) the whole game was a hallucination caused by this horrific operation. Now that you've escape the trap of your mind, you must figure out what really has happened to you, while fighting off flash backs induced by your hacked up brain. It may not be as suspenseful as a Stephen King novel, but it gets the job done.
With a fine tuned user interface and more mutated looking evil to fight, this should fill your daily gore and FPS gameplay needs. I would give it a look and support one of the best small developers the DS has.