Previously the Mac OS X backend indeed used its own machinery to create PDF files. Recent versions of the backend in SVN, however, use matplotlib's pdf backend. So the problem should go away if you use matplotlib from SVN.
The Mac OS X backend itself can actually be fixed to use vector graphics on screen instead of bitmaps. That will need some time, but I'll get round to it one of these weeks.
--- On Sun, 6/28/09, Jouni K. Seppänen <jks@...397...> wrote:
The file you sent was not generated by the pdf backend but
by "Mac OS X 10.5.6 Quartz PDFContext", which probably means
that the OS X backend saves pdf files using the OS X machinery
and not the pdf backend. Indeed the formulas look like bitmaps.