On my layout I have a couple of graphic files (jpgs) I've added to the layout. When I export to pdf, the graphics are dropped from the pdf file that is created (that part of the layout is just blank). This is an mxd I have successfully exported to pdf in the past, but doesn't work now in either 9.3 or 10.
Images dropping out on export is usually related to your default printer page size. See the following knowledge base article for some suggestions:
What we've done in this situation is compose and export the map at 50% of final print size, output at 600dpi, and then double the scale in Adobe reader/acrobat when printing. It took some careful calculating to get the font, graphic sizes, etc. correct but worked.