Following is from a U.S. laywer, for those who needs to go back china, if you are in"sensitive fields, be prepaird for " check on Technology transfer ". My girlfriend is a hardware engineer working in sillicon valley and still waiting for the clearance, it has been 2 months and the words I got from washion visa office is "at least few more months"....
http://immigration-information.com/showthread.php?t=1631
I doubt that this is the type of security check that we are seeing in other countries. The big issue with Chinese applicants is technology transfer. If you are involved in work that can be classified as "dual use" (that is, both civilian and military), then your company will need to obtain an export license for you.
Please keep in mind that this includes the techniques that you use to produce a particular product, as well as the product itself. For example, a software engineer working on merchant server software might not think of his or her work as falling into this category. If that engineer deals with 128 bit encryption, however, he or she will need an export license to work for the company.
You appear to work in a sensitive area. If in fact you do, then this is the most likely reason for the hold up. I have told clients repeatedly that they must always check to make sure that they don't need export licenses, yet many of them have gotten caught up in this same issue because they failed to check before leaving the U.S.