The function to delete the selected page (or pages) from a PDF file disappeared from the Edit menu of Preview in Snow Leopard. Even worse, it was replaced (using the same Command-Delete shortcut) with a command that moves the entire PDF document to the trash.

You can, however, still delete selected page(s) through an undocumented keyboard shortcut. Hold down Shift-Command-Delete with one or more pages selected, and only those pages will be deleted from the document. Page numbering, however, does not update to reflect this deletion until you (re)save the file (in 10.5, the numbering updated instantly upon deletion).