Prentice Hall did a low first printing because they were worried about the online book cannibalizing sales. At this point I would never go back to the old model of print publishing. All of my future books will be electronically published on my site first, and will stay on the site. I get extremely valuable feedback during the development of the book. I've never had any useful feedback to speak of from the so-called technical reviewers hired by a publisher, but I got an endless stream of incredibly valuable corrections from readers.
Readership is built throughout the development of the book.
Books can be adopted by universities before they are printed. When the book is actually printed by the publisher, the university has already decided to use it for their classes, and they tend to be reluctant to change. On a more practical level, folks who may not have money now but eventually will get to know you, and when they can purchase they're much more likely to purchase your book. In general, the fallout from doing this has been all positive and nothing negative. Upon closer inspection, though, what one discovers is very interesting.