Nice catch ShawnZ! who posted about the following article in Back page News.
It seems that the Office 12 Beta serial codes handed out for free by Microsoft work on any version of office. However, you can"t be too sure if it will expire in February or not.
A legit key-holder myself, I have been unable to verify the claim but comments show that this is indeed the case. I suspect that Microsoft may have a hard time blacklisting the block of keys they handed out, partly because its a different situation when you compare it to the infamous Devilsown Windows XP cd-key they leaked weeks before Microsoft RTM"d it.
Microsoft was unavailable for comment at the time of this posting.