I want to buy WoW, what's the real deal with this "free month"?

I mean, ok let say I buy Vanilla WoW... yay I got a free month, but if I instantly buy Burning Crusade, Wrath of the Lich King, and Cataclysm... and upgrade the account the same day... this "Free month" will still be there?? (Stupid Question I know..)