I had never thought of The Waste Land as an archive prior to reading Foucault, but when I consider the poem in that light there really isn't a better term that can be used to describe it. According to Foucault's definition, an archive "...unites in a single "volume" a series of linguistic elements that derive from existing books and that are, by virtue of their specific documentary character, the repetition of things said in the past" (105). The Waste Land is the only poem I can recall reading that appears to be more like a catalog of other ideas than an original text; it "cataologs" ideas, phrases, and direct references from world history and other works of literature throughout, and it's one of many reasons I enjoyed reading the poem. I loved trying to figure out where certain references were being pulled from.
Archives are also used to preserve history and literature, and The Waste Land achieved that by reimagining and throwing together older texts, presenting them to a new audience. No work of literature is original, as each new piece either directly or indirectly draws upon previous works for inspiration. T.S. Eliot seemed very aware of this in his writing of The Waste Land. He purposely sought out references for use in his own work, which - strangely enough - makes the poem original, from a certain point of view.