Lots of games on the iPhone skew more casual, both because the touchscreen doesn't exactly make for precise controls all the time, and also because a lot of developers are aiming for the iPhone's much wider audience than most gaming platforms. But 1-bit Ninja is obstinately the exception to this -- it's unapologetically retro and decidedly hardcore. The nearest reference is Super Mario Bros, but not Super Mario Galaxy or the more recent 3D interations. Nope, this one goes back to Super Mario Land on the GameBoy, when Mario could only move forward, and had to make some spectacularly timed jumps to explore the land and fully discover its secrets.
1-bit Ninja isn't just a 2D platformer -- it does have a really wild 3D angle (accessed by swiping the top of the screen) that will show off some new heights or extra secrets to find. But especially early on, that mode's somewhat underused, with most of the gameplay sticking with the 2D realm.
Still, with over 20 levels to play, challenges to beat (too bad Game Center isn't included -- seems like it should have been), and lots and lots of secrets to find, along with a great stylistic design and some cool chiptunes to play with, 1-bit Ninja is an excellent title. It's available for US $1.99 in the App Store right now.