I recently read about a device that reported its "minimum CPU speed" (which was 50MHz; max was 1500MHz) and I wondered if my MacBook Air (and other devices like iPhone/iPad) slows down the CPU when not plugged in.
Yes. Intel calls it "SpeedStep." Variable CPU clock speeds that adjust to conserve power based on demand. The feature is engaged whilst the device is on battery power. All Apple products support this feature (it's actually supported at the hardware level and the operating system's kernel) but since desktop devices don't have batteries, you'll never experience it on anything that isn't mobile.
Your MacBook Air supports SpeedStep, as does the iPhone, iPad, and the traditional MacBook Pro.
More info on SpeedStep: http://en.wikipedia.org/wiki/SpeedStep#Mac
I haven't seen any evidence that the MacBook Air scales down its CPU to preserve power under normal circumstances.
That said, when your battery runs very, very low, you will probably notice significant slowing. Whenever I play a high-quality game on battery power, the game inevitably slows to a crawl when my battery drops below about 8% capacity. Plugging in will immediately increase performance, so this is clearly the intended behavior.
@Siddhartha's statement regarding iOS devices rings true for laptops as well, since they are both designed to be used primarily on battery power.