The most common device used for reading a digital book inthe US is a PC

"Roughly 2% of American book buyers over age 13 are active ebook users, meaning they obtained an ebook or a reader device in the last year. About half of those were first-time ebook buyers, so the usage of ebooks has probably roughly doubled in the last year. BISG is doing multiple waves in the survey, and says it found a 25% increase in ebook usage just over the holiday season, so it was a pretty good Christmas (and Hanukkah) for ebooks.
The most-used device for reading an ebook is a personal computer (47%); Amazon Kindle is number two (32%), followed by Apple’s iPhone and iPod Touch (21%)."
Source:  Consumer Attitudes toward E-Book Reading Report, by the Book Industry Study Group, reported by the Digital Book World blog, 25th February 2010