The tablet market is still a one-horse race, but e-readers continue to ramp, and Walmart wants another piece of that precious pie at big box retail. Starting this week, the online retailer will deliver shipments of Amazon's mighty Kindle 3G and ad-supported Kindle to over 3,200 Walmart stores across the US, where it will join the Nook and Kobo e-readers in competing for shelf space. Could this be the beginning of another e-reader price war? Perhaps not yet, but it could be one more retail channel for Amazon's ethereal tablet.