A link taken from the Wikipedia article on Underworld: http://www.lichtensteiger.de/WTCunderworld.html
Here DeLillo explains how he chose the title of the book:
"While I worked on the book, I gradually compiled a number of titles. I first hit upon Underworld when I started thinking about plutonium waste buried deep in the earth. Then about Pluto, the god of the dead and ruler of the world. New connections and meanings began to suggest themselves, and I recall drawing a circle around the title Underworld on a page filled with prospective titles." — Don DeLillo to Jonathan Bing, 1997
We've already been introduced to the theme of waste. Nick Shay works in waste management and the fans were throwing scraps of paper, anything they could get their hands on really, down on Pafko every time he comes close enough to the wall. I'm not sure yet what the waste theme means, but I figure it's always a good idea to know about themes at the beginning as it makes instances of them easier to pick out. I'll definitely be on the lookout for when waste comes up again...
Your entry brought to mind a paragraph on page 88:
"The Jesuits taught me to examine things for second meanings and deeper connections. Were they thinking about waste? We were waste managers, waste giants, we processed universal waste. Waste has a solemn aura now, an aspect of untouchability. White containers of plutonium waste with yellow caution tags. Even the lowest household trash is closely observed. People look at their garbage differently now, seeing every bottle and crushed carton in a planetary context."
Clearly, the idea of waste dominates this entire paragraph. Then, in the next paragraph, it talks about how the world heals itself on its own from the waste, which is an interesting thought. The plutonium waste (as mentioned in both your article and the novel itself) is definitely going to be a big part of the book, I think.
ya, and the plutonium waste specifically relating back to Pluto allows for a lot of parallels between Underworld and GR. Pluto, which we learned from the GR Companion is the planet of war and death, is also the root of the word plutonium. Somehow, the war relates to waste, though I'm not in a position yet to decide how.