According to a leak spotted by Ars, if you buy Microsoft Office 2007 anytime between March 5 and September 30 of this year, and you activate the product and save your dated receipt, you can get Office 2010 for free.
Ars has more details, but slow down, there's a catch: The info was yanked after it was published, and Microsoft now won't say whether it's legit or not. I'm guessing it is. Whether you believe it or not, play it safe and don't buy any version of Office until March 5. [Ars Technica via TechFlash]