Over at MacOSX Hints, a poster has figured out how to rotate a single page in Preview without affecting any other pages in a PDF file. The secret lies in the option key. By holding it down before clicking either Rotate Left or Rotate Right, you limit the rotation to the currently displayed page rather than the entire document. This is particularly convenient when you want to rotate a single portrait figure into landscape or vice versa.
For those of you looking for the Rotate Left and Rotate Right icons while displaying your PDFs, be aware that you may have to resize your window to reveal the entire tool palette.