If you don't like flashbacks, have a spoon full of sugar ready to make 'em go down easy when you read White Teeth—they sure are plentiful. The way that we understand the past (and the way that the characters do) is critical to making anything out of the novel. Luckily, White Teeth's epigraph does an excellent job of introducing us to how Zadie Smith wants us to think about the past: "What's past is prologue." So, the past is always the set-up for what comes next, for the present. The Joneses' and Iqbals' histories seem to color most things that they do, think, and feel. At the very end of the book, for example, we find out that we have misunderstood what happened to Archie at the end of WWII. And with this new information, a lot of things change.
As much as the younger generation would like to be free of the past, they are stuck with it.
Irie, Magid, and Millat are able to live in the present and not worry so much about the past.