Here's the exact message I received (note - I replaced my phone number in the URL with 'XXX'):
You qualify for a $1,000 giftcard. Less than 200 remaining.  Goto http://giftcardonmobile.com/?p=XXX enter code 1922 to verify. Optout at celloptout.com

Went to celloptout.com just to see what website was like.  It was a single page prompting you to enter your phone number if you wanted to be removed (i.e., opt out).  I didn't enter my number as I'm guessing that was a spam text message, and entering your number confirms the text message was sent to a legitimate cell number.