NIS 2010 registration key ... backward compatibility?

I've seen several posts that confirm that a NIS 2009 registration key will work if you upgrade to NIS 2010. 

 

However, how about the reverse: Can a 2010 key be used on NIS 2009?  I'm seeing some good deals on NIS 2010 (1-User/3-PC for under $30, no tax, free shipping), but for now I want to stay with 2009.  I'd like to buy NIS 2010 and use the key, not the CD. Is this possible?

 

Thanks ...