Carla Montori spoke about the Google digitization initiative at the University of Michigan at the LIBER Think Tank on the future value of the book artefact and the future value of digital documentary heritage [pdf]. She reported an interesting finding: experience of the digital version of an item is creating demand for the physical item. One can imagine various reasons for this. For some folks and for some uses immersive reading may not be congenial in the digital environment. There may also be cases where there is interest in the original artifact. And there may be other reasons.
It does mean that it is unlikely that digital materials will be a simple substitution for print. This has implications for physical collection management. However, this does not necessarily mean that libraries retain all that they digitize on local shelves. It does mean that what is digitized may have to be quickly accessible within some framework of predictable and reliable delivery, whether that is local, or, increasingly, within a shared context.
Update: Check out the comment by John Wilkin. In my own experience, which is nothing more than that ;-), I will sometimes go to a print version having come across something first on NetLibrary or Google Book Search. It will be interesting to see what behaviors do emerge when we begin to have data to work with.