I believe independent assortment occurs at anaphase II of meiosis. At this point, genetically variable (via crossing over at prophase I) homologous chromosomes have already been separated (during anaphase I). The law of independent assortment states that during anaphase II, the separation of these non homologous chromosomes into sister chromatids may be assorted into daughter cells randomly, without influence of any of the other chromosomes.
...I could be wrong. Can anyone else confirm this?