Sure, it has a similar screen resolution (2048 x 1536), the same iOS 7, front and rear cameras, and it even has the same incredibly powerful A7 chip (4 times as good as its predecessor). But it turns out that the chip in the iPad Mini isn't running at the same clock speed as the iPad Air. The larger tablet’s A7 processor runs at 1.4GHz, while the iPad Mini is showing up in benchmarks as running at 1.29GHz.
That's the same speed as the A7 chip in the iPhone 5s, which might be a clue as to the difference. I would bet that Apple set the processors in the iPad Mini and the iPhone 5s to run slower in order to reduce the chances that the devices will overheat when running under a heavy load. The iPad Air, on the other hand, has a larger mass to act as a heat sink. The increase in waste heat probably won't be noticeable.
You can find the iPad Mini on the Apple website and on a few other retail sites. I'm not sure if it is in stores yet.