Check out this thread on the Barnes & Noble forums: According to reports, ebooks were the #1 selling format in February 2011. If you look at the first post in the thread, which lists the millions of dollars of each format (ebook, hardcover, trade paperback, and mass market paperback) sold in February of every year since 2006, you'll see that ebooks have risen dramatically in the past two years, while hardcover has taken a serious hit this year. And if you do the math, you'll see that there was actually less in sales in February 2011 than February 2010 — the spending has just been redistributed. In other words, a LOT of people who were buying hardcover in previous years are now buying ebooks.
I just wish there were some statistics here about the number of copies that were sold of each format. eBooks usually cost less than hardcovers, so $90 of ebooks is probably at least twice what $90 of hardbacks would be.