Here is an interesting article on tracking down the source of spam:
It is very normal that more than 1/3rd of the domain names we see each day in spam messages come from China. When one also considers the many “.com” and “.ru” domain names which are also hosted in China, the problem is much worse. More than half of all spam either uses domain names registered in China, is sent from computers in China, or uses computer in China to host their web pages. The numbers above look much higher than half, but these are numbers about spam DOMAINS, not the actual number of spam messages. Some non-CN domains send a disproportionately high number of messages.