There's a new seemingly important paper around but it is behind a paywall so I can only make a shallow comment: David Reich, Kumarasamy Thangaraj, Nick Patterson, Alkes L. Price & Lalji Singh, Reconstructing Indian population history. Nature, 2009.
A divulgative review can be read at Science Daily.
Apparently the research has found or confirmed two important elements in the Indian genetic background:
1. That there were two ancestral populations in the subcontinent: a northern one (ANI), not very different from Western Eurasians and a southern one (ASI) totally unique to India and whose only pure remainder are the Andamanese. Otherwise both distinct ancestral populations are nowadays found mixed at various apportions.
2. That tribes and castes are really not that different, with castes seemingly being formed out of recycled ancient tribes. This means that there are many tribal/caste founder effects that make each of these groups rather unique and inbred, with likely relevant health consequences like those found among other similarly isolated populations, such as Jews or Finns.
Nothing too new, because I do have the feeling that this was vaguely known but never really established in such categorical terms. For example, in some previous global studies South Asians have often appeared as a mixture of two components: one the same as West Eurasians and another unique of them. Even the anthropometrists of old used to talk of "Caucasoid" and "Australoid" (or sometimes "Veddoid") Indians, even if we know now that the use of the term "Australoid" in the past was all but clarifying, mostly meaning anything Eurasian that is not specifically Western or Eastern.
For me, this confirmation of the existence of two distinct ancestral populations, suggests that they formed in the early period of Eurasian spread of humankind, before what we conventionally call now "races" formed. As West Eurasia was colonized since c. 50,000 BP, that provides a most recent time limit for such population divergence, because it must have happened before, probably quite a bit of time before, some of the ANI peoples migrated to West Asia, North Africa and Europe. The "Indian remix" must then have happened after this westward migration.