technically you can but consider how big walmart stores are. Sometimes they'll have the game on the shelves prior to release date. You try to buy it and the computer denies you. But at 12 am of release day they will sell it.
However if it is NOT on the shelves forget about it. They won;t go and look in the warehouse for you. And if they do they probably wont find it.
White Horses + Swirly Horn + Wings = Joy and Peace
i have done this a number of times since i work from 4pm to 12am. the 24 hour walmarts will be stocking the items at 12. sometimes the games are still in the boxes. Expect to ask specifically for the title and wait till they find the right box. Once, when i bought rock band 3, the cashier said "its around here somewhere" and let me open boxes till we found it. or they may release early, but thats more dependant on the cashier.
I work at Wal-Mart and it depends on the store for the most part. Ex: At my store we don't do midnight releases because we actually close at 12.(We caught pure hell for this around the CoD MW3 release) However, at large Wal-Marts that stay open 24 hrs, they usually will start selling it upon midnight. If it's a well known anticipated release like GoW3 and Halo was, they will probably have them ready by 12 if they get enough time between customers to have them ready. However, it is near impossible to sell most games before release because even in the instance the cashier does not refuse to sell it before 12, which the cashier should not sell before the release date, with most games in the system the register will refuse the sale until the release date and this could only be overridden by management, which the first thing they will ask is why you are selling merchandise before the release date.
Also, as the above poster said, there is also a good chance it is in freight hell, meaning that in this instance you may have to wait while the cashier(maybe someone else but if anything like my Wal-Mart only one electronics associate available during this time period) sorts through the boxes to find which box has that item.