In a recent Nature paper by Lalji Singh et al., they found genetic evidence for three ancestral migrations into India (fig 4 from the paper). The oldest being out of Africa and represented today by the Onge people of the Andaman Islands, followed by introgression of the ancestors of the modern South Indian (ASI) populations today followed much later by introgression of the ancestors of the North Indian (ANI) populations.
They further note that almost all Indians today are admixed from the ANI and ASI ancestral populations.
They also find evidence for many founder events took place followed by population expansion without much gene flow between groups.
Today's South Indians often speak a Dravidian language while the North Indians often speak an Indo-European language.
There is a free Nature podcast with a segment devoted to this paper. Skip to about 24:45 for the 6.75 minute segment.
Ref: Vol 461|24 September 2009|doi:10.1038/nature08365