Apple purchased a whopping 349 million micro electro mechanical systems (MEMS) microphones last year to put in all of its various iOS devices, which means it is now the biggest purchaser of such microphones in the world. That number grew 173 percent from 2010 to 2011, putting Apple up over Samsung for the most microphones bought (and presumably installed and used).
These specific microphones represent the three microphones found in each iPhone 4, 4S, and iPad 2. One of the mics is found in the headphone cable included with each device, and two are on the device itself, with one of those used strictly for noise canceling. This is why Siri works so well at "hearing" even in relatively crowded environments, because there's an extra microphone on board to listen for noise to remove from the incoming signal. It's also worth noting that the iPad doesn't have this third mic, which might be one reason why earlier generation devices haven't been marked for Siri use.
The next-gen iPad will have all of these components too, so Apple isn't done buying up these parts. In fact, if we see both a new iPhone and a new iPad in the next year, Apple could cause yet another spike in the sale of these mini microphones and move even higher up the chart of consumption for this specific part.