There is no direct method of converting pdf to ppt, but Adobe Acrobat 6 Pro can convert pdfs to Word documents, with a mixture of graphics and editable text. You can convert to powerpoint from there. (req purchase of Acrobat Pro 6)
Also Adobe Illustrator and Macromedia Freehand can open and edit pdf files if you have these apps.
You can then drag and drop elements back into powerpoint. Or from illustrator you can export paths to wmf (windows meta file) file format which are compatible with PP.
If you just want a straight import of pdf to powerpoint without editability, open your pdf in the Preview application and Export each page to JPEG which can be placed in powerpoint with the 'insert picture from file' command. (free)