Well, it's important to remember that wage labor was not all that common in the South at the time. Very few people actually worked for wages. There was lots of land in the South to work on, even if you did not own it. Some people have made the claim that slavery delayed the South's adoption of a wage labor market, but personally, I don't find this argument very convincing. Furthermore, plantation owners occasionally lent their slaves to poorer whites to bring in crops and whatnot. Most whites did not want to harvest crops like the slaves did and did not mind that slaves were doing that work. Instead, it was fairly common for lower class whites to be an overseer on someone else's farm or plantation.

Whether or not all whites benefited from slavery is another matter. Since most whites did not own slaves, they were not that affected by slavery as an industry. There were those planters who lent slaves to others, as I said, but that can only be judged on a case by case basis. They certainly received a psychological value from having slaves to look down upon, but then slaves often looked down on poor whites in return. Plantation owners occasionally went to great lengths to share the benefits of slavery with everybody else, holding giant BBQs and parties that the whole county was invited to, but this didn't contribute a whole lot to the material well-being of poor whites.

Now, one could argue that plantation owners could grow a heck of a lot more cotton, due to the concentration of slaves on their estates. This is partially true, but it's also important to remember that big plantations and small farms grew very different crops. Plantations were almost entirely devoted to cash crops like sugar or cotton, while small farms mostly grew subsistence crops with a bit of cotton on the side. So, there wasn't all that much overlap. Furthermore, small farmers frequently sold their food to big plantations, so rather than competing, plantations were providing business for small farmers.

I approach this question from a slightly different angle here. http://www.reddit.com/r/AskHistorians/comments/1t66hq/how_did_poor_southern_whites_feel_about_slavery/ce4y7mb

Some sources: Bertram Wyatt-Brown, Southern Honor, Stephanie McCurry, Masters of Small Worlds, Owsley, Plain Folks of the Old South

In the recent book The Half Had Never Been Told, the author makes a convincing case that all whites benefitted because, in slave states, slavery was such an important part of the economy that all businesses were tied to the fruits of slavery. Slaves meant an infusion of capital to the region. Everyone in a cotton-producing region deals with cotton in some way, either selling to the plantations, or doing something with the dirt cheap cotton they produced. Virtually all the cotton mills in Massachusetts and London, etc. that represented the flower of the industrial revolution, were picked by slaves. While much is made of the increase in productivity in these mills due to new technology, the author points out that the raw material for these mills was created due to increasingly cruel methods of extracting labor from slaves--mid 19th century slaves picked 2.5 times as much cotton per capita as 18th century slaves due to a more brutal way of treating them, and free people couldn't even come close in amount pickled until the mid-20th century with vastly superior technology and modern agriculture.