Obvious scam.  Fone Finder USA indicates that this is a Cingular subscriber in Ramona, California.  (Cingular is now known as AT&T Mobility.)  Notifying Walmart via eMail may be helpful.

Got text msg - You've been selected to win a FREE Walmart Giftcard worth $1000! Enter the code 'FREE' at www.walmarat.com.wlat.biz to get 1 now! Only 118 left. Txt STP to stop.

Heard from a friend - if you do txt STP, you'll be charged $39 through your cell phone carrier.

I got a text saying I won a giftcard from walmart. Spammer I'm pretty sure