Mac or Windows?
If Windows it could be a conflict with Acrobat. Do you also have Acrobat installed? Have you tried trashing your Illustrator preferences? I'm only throwing out random suggestions based on what I've read before as troubleshooting steps but I could be way off the mark.
I have windows, I have acrobat installed, this problem started a while ago, and i was ignoring it, but now I need it desperately. How do I trash my Illustrator preferences - same way as InDesign?
I think I just trashed my preferences, when I opened Illustrator again I held down CTRL SHIFT and ALT. My welcome screen didn't have any of my recent files. It still crashed when I tried to save to pdf.
I think the problem started when I recieved a file from a client, that he recieved from his printer in China. When I tried saving those files, there was chinese in my dialog box, and the files were crashing, and in later files I still had chinese, I got rid of the chinese somehow, but it still crashes.
could be a font issue that's corrupt somewhere.
Check out this document and see if anything applies (you may have to scroll down a bit)