This is a popular question, see:
http://en.lmgtfy.com/?q=is+motherboard+serial+number+unqiue[
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gave me over million answers; I did not see a definitive answer so far and I don't want to examine them all :-), but there are indirect signs showing that nothing is designed to guarantee world-wide uniqueness. In sign is the information, that a format of unique id is different for different manufacturers. If there was a global organization regulating so many motherboard manufacturers, the IDs would take a uniform format. Another reason for not having such a global system is this: who is interested of such global uniqueness? Hassles and costs for manufacturers are obvious, but what would be the benefits? So, I don't believe you can rely on that.
Other identifications include MAC address and CPU id.
Let me tell you: if you're developing a licensing schema that requires your installed software to be operational only on a single uniquely identified computer which would loose the rights to use your software on hardware upgrade, this would be not fair to your customers, in my opinion.
—SA