My colleague Joel Geraci, Adobe's Acrobat Technical Evangelist, just posted an excellent blog entry about the new Enhanced Security model and Privileged Locations from Acrobat 9. It's extremely important reading for anyone working with interactive PDFs.
Quoted from http://blogs.adobe.com/pdfitmatters/:
Enhanced Security: Privileged Locations
Security is often at odds with usability. Adobe Flash began addressing this problem several releases ago by implementing and standardizing on a cross-domain security model that has evolved over the years into a robust, secure solution. By providing controls for who may receive data from whom, Adobe Flash can power rich Internet applications that are safe and extremely flexible.
An excellent explanation of the cross-domain security model can be found here