Straight talk promo codes are pretty rare. So I am always a little surprised to see them when they appear.
There appeared to be a temporary glitch in the March 2018 code, but it turns out to have been canceled for "vendor legal reasons". I'm not sure what that means, but it may have something to do with the fact that Straight Talk brand is a collaboration betweeen Tracfone's parent company, America Movil, and Walmart. So this code's may lie somewhere in the terms of that relationship.
They have had a few promo codes before, so I hope this gets sorted out eventually.
Get 30% off your order when you purchase any phone or phone bundle with airtime; at StraightTalk.com w/ promo code STRAIGHT30. Max Savings $100. Valid 2/1-2/28
The fine print appears to say that it doesn't apply to iPhone purchases. I haven't tested it on everything they offer, but it seemed to work on all the other smartphones I checked.
Update 8/4/2013: No longer valid.