Walmart does sell some seemingly identical products as other retailers for a lower price, when in fact the item is of lesser quaility. The manufacture may use cheaper materials in order to keep costs down.
I don't know if they do this with phones. The Walmart version of a product can usually be differentiated by the part number. There is usually a letter different than the higher quality product. I believe the normal part number for the Droid X is DROIDXVZW. If the phone at Walmart has the same part number, it should be the same as getting it any where else.