I noticed that DeLillo uses the present tense to describe things that take place in the 50's. The "present", or at least the parts of the book that are most like our current world, he generally describes in the past tense.
I was wondering why he did this. Does it say that the past, the 50's, is somehow more alive and real to us than what we think of as the present? I don't really know. I thought about Gravity's Rainbow and how it's like you are reading the whole book from a later time than WWII (maybe). Perhaps it's like we are reading the book from the 50's and looking forward...but that sounds silly so maybe not.
One other note on tenses. On page 275 Nick is speaking in first person in the present tense. That is also supposed to be a number of years before the 90's, right? I'm not sure...but if it is "older" than it would explain the tense. I'm thinking that the 1st person somehow makes Nick seem younger.
It's interesting that the time when the cold war was going on is the section that's in the present tense. Maybe since that's the focus of the novel, it's considered the "present," and everything else the past? I'm not sure how much sense that makes, but it's one theory that I had.