Blizzard started to provide free trial promotions of Starcraft II: Wings of Liberty as this ad campaign by EVGA seems to confirm.
EVGA is offering an activation key code good for 7 hours of continuous gameplay, or 14 days to play StarCraft II: Wings of Liberty (whichever comes first).
This free trial is available when you purchase eligible EVGA GeForce® Graphics Cards purchased on or after July 27, you will have the option to collect a code on your product page (product registration is required).
Simply click on “View Details” and scroll down to promotions. You will see detailed instructions there.
Terms and conditions apply. Go to www.GeForce.com/TryStarCraft2.