As Walmart's first dance, i would be hesitant about ordering through them (especially with the "Self Check Out" system they are bringing to the table this year. The $10 discount might not be worth the headache...
I hadn't heard anything negative in regard to problems last year when they sold the iPhone 4S at launch. They were sold out like everybody else except HHGregg stores that only cater to Verizon phones.
I know with the iPad, they sell all 3 models, but I'm not so sure in regard to the iPhone. I went on their website and could only find the 16GB model, even for the 4S, but that's not confirmation, I know.