Well, my area is more "Boondocks" than "Podunkville"

, but I have seen Wal-Mart be the impetus for small businesses falling. Conversely, I have seen a death row prison as the impetus for Wal-Mart investing in the town, as well as McDonald's, Taco Bell...etc.
In regards to the job displacement/replacement theory, that may vary based on the area. You specified 30K as poverty range, I'm assuming that would be the lower end of the poverty range in most areas. The poverty range in my area is not 30K, that's a
GOOD paying job. Most people here make right at or under 15K a year. For those of us who make more, it's either travel for a job in the city or get an education that will allow you to manuver through the changing workforce requirements.
As for the jobs that were displaced and replaced by Wal-Mart of which I have specific knowledge, the pay was comparable. What I found interesting was that the majority of new Wal-Mart employees found Wal-Mart to be a better job from the one lost as the previous employer didn't offer any benefits. However, this may be an anomoly specific to my area, so I wouldn't apply it to the larger Wal-Mart theory.
It's interesting what can change society and the changes society will allow for to accomodate such things, isn't it?