Two books have recently been published that I want to read.
The first of these I took delivery of instantly in PDF form, and a paper copy will make its way to me in due course so I have it as a handy reference.
The second of these is going to take 12-14 days from Amazon (5-7 days if ordered through O'Reilly's UK distributor Wiley).
There's been a lot of talk on David Heinemeier Hansson's blog about PDF publishing, but to me it's simply a way of getting me the information I need quickly. Normally, I want the paper copy too.
Another small but significant benefit is that PDFs weigh very little! Having just taught a Rails tutorial at XTech, I can vouch that there was no way we wanted to carry around 30+ copies of the Rails book on the offchance attendees wanted to buy it.
Whereas some publishers have experimented with certain product lines available only as PDFs, I can't quite shake the feeling they're offering an inferior product in which they are not confident enough to commit to paper.
My suggestion is that PDF and print is not an "either/or" choice, but an "and/or".