Many of the tools currently assume that PNG numbers are strictly numeric and monotonically increasing. Perhaps this should be changed (I would argue yes :) ), but that does create complications with how page renumbering should work, for example.
I know of one fairly prominent commercial digital library (Eighteenth Century Online) that made the decision that the issues with using page numbers as unique identifiers are sufficiently hairy that they went with sequential *image* numbers (which are unique), and the database maintains as metadata the page number that goes with each image number (which may not be unique, sequential, numerical, or even present). This seems to be the route DP is headed with the proposed "metadata collection" round. -- RS