Generally, the iPhone and the iPad get all the noise, and why not, they're sexy, but the iPod Touch has been a stealthy money-maker for Apple, more than an iPod and less than an iPhone, it's been selling huge and quietly.
In February 2010, BusinessInsider.com featured it on their always interesting Chart of the Day:
Twice last year, Apple gave more concrete information about iPod touch sales. Since then, it's clear that while the iPhone has grown faster over the past year, the iPod touch has continued to sell well. (During the December quarter, for instance, iPhone shipments grew 100% year-over-year to 8.7 million, while iPod touch shipments grew slower; 55% year-over-year, Apple said.)
Bottom line: It's obvious that the iPod touch has been a huge driver for Apple's iPhone OS, its mobile apps ecosystem, and app sales -- especially games.