Has Wal-Mart Buried Mom and Pop?, PDF from the Cato Institute:
Our research suggests that the popular belief that Wal-Mart has a significant negative effect on the size of the mom-and-pop business sector of the United States economy is statistically unfounded.
After examining a plethora of different measures of small business activity and growth, examining both time series and cross-section data, and employing different geographic levels of data and different econometric techniques, it can be firmly concluded that Wal-Mart has had no significant impact on the overall size and growth of U.S. small business activity.
There is no question that Wal-Mart does cause some mom-and-pop businesses to fail. However, those failures are entirely compensated for by the entry of other new small business elsewhere in the economy through the process of creative destruction.