Interesting statistics from O'reilly:
We've been selling PDFs of our books on oreilly.com for several years, but this summer began selling "ebook bundles" of many titles, which include PDF, EPUB, and Mobipocket versions. Here's some weekly data (I can't share the vertical scale) on the relative breakdown of actual downloads from those bundles (PDF, Mobi, and EPUB are Light, Medium, and Dark respectively). PDF is still the format of choice for most people, though EPUB is getting respectable usage, with Mobi in third:
The numbers at the bottom are weeks (200901 is the first week of 2009). This is only among titles offered in all three formats -- the majority of our ebooks are currently still only available as PDF, though we expect to release several hundred more in bundle form over the next few months (not that you should wait to buy of course -- you'll get all the formats as they come available ...).