So I have had this problem a few times when exporting from illustrator as a pdf, this hair line shows where the bounding box would show around the edge of objects. Its just a fustration and it always seems to happen on a deadline! Now I used to use the PDF/x-1 format when I was having the problem.
Use the PDF/x-4:2007 format. This format handles transparency much better and no hair lines!
This problem relates to the use of texter on vista systems. So after three weeks of pulling my hair out shouting at Vista, banning the keyboard, trying to fix, trying to live with just trying not to put the computer through the window. Finally today I have solved the problem.
The problem:- The keyboard was not functioning correctly on my vista system, shortcuts (hotkeys) did not work, some keys did not work, life was hell!
The cause of the problem:- texter (great little free application that I discovered on lifehacker) http://lifehacker.com/software/texter/lifehacker-code-texter-windows-238306.php
The solution:- Uninstall texter. Now to do this you will need to delete the texter folder in c:\programs. But before you delete the texter folder make sure the program is not active.