You hit on the two most obvious connections: the type of food the poor can afford to eat and food as a cheap status/feel good fix.
Regarding the former, it's obvious that the working poor are going to have a steady diet of carb-heavy staples like rice, pasta, cereals, beans, fruit and the king of white flour - breads. Their meat intake is going to be limited to cuts or ground blends with heavy fats like "75% lean" hamburger, chicken thighs and legs and other less glamorous cuts.
As to the latter, the easiest way for a poor parent to make their kid feel "normal" and have the social status of other kids is to take them to McDonalds or some other fastfood or mall food establishment. Like Kelly said, it's cheaper than buying some CDs or fancy shoes, and the kids relate the experience to being like every other kid in their class who has bought into the same thing.
This mindset follows the working poor even as they climb the socioeconomic ladder into the alleged middle class, but I'd suggest for a different reason. While the parents may now be able to afford a more balanced, reasonable diet, they are probably working more and more hours at the office, especially if they advance to white collar, salaried positions where the impetus is to slave away 50+ hours a week and be hooked up by email, cell phone, pager, Blackberry or all of the gadgets combined. That leaves little time for an educated trip to the grocery store, co-op and even less time for preparation, so the families fall back on the same tired fat/carb staples that are easy to fix or succumb to the restaurant industry or homecooked fast food like frozen dinners and snacks.
- Deborah