About a third of the print books in WorldCat have ISBNs.
Brian Lavoie provided me with this number based on the data extracted from WorldCat to support his joint work with Roger Schonfeld of Ithaka on the systemwide view of library collections. (Their presentation to CNI is available [ppt]; an article describing their results is forthcoming in the Journal of Electronic Publishing.)
At the time of that analysis, January 2005, there were 32 million print books in the system-wide collection represented by WorldCat. 10.5 million had ISBNs.
This is an analysis that would be worth repeating at some stage, given the increasing rate of growth of WorldCat.