I think it is pretty obvious that talent does exist. It is equally obvious that talent will not be realized unless one actively develops it, and this is exactly were the environmental factors steps in.
As for RiskyP's question of how genetics might give rise to talent: Our genes pose limits on what our body and brain can do. Two newborns may be born at the same height and weight, yet one might grow to be 6'6'' and the other to a mere 5'6''.
Where did this difference come from? Assuming both people had adequete nutrition throughout their childhood, it all came from their genes.
The same is true for the connections in your brain. It is your genes which determine the general direction in which these connections could develop. Only the degree to which this potential is realized, depends on the environment.
There is ample proof of this. The most striking example is the researches involving seperated identical twins: Each twin had exactly the same genetic markup, but they lived in totally different environment (often in different countries with completely different cultures).
Despite the environmental differences, the two twins developed many similar traits - from the way they dressed, to their chosen occupation, to even the personality of the girlfriends they chose!
So don't underestimate the power of genetics. It is a very powerful factor in shaping our skills and personality.
That being said...
I do agree that competent piano playing does not require special talent in music, any more than doing basic arithmetic requires a special talent in math... Nearly every 6-year-old child can master both with relative ease, which seems ample evidence to this claim.
However, I do believe that at the higher levels, talent is essential.
Every child can work to become a good piano player, but playing at the level of the top concert pianists requires talent.
Every child can work to become a good amateur chess player. Being a Chess Grandmaster requires talent.
Every child can work to run the 100 meter dash in 11.8 seconds. But cutting off those final 2 seconds to break the world record (around 9.

requires talent.
And of-course, a child with talent would progress much faster from the beginning. There are ample examples of this in the biographies of pianists, scientists and inventors - I think this is evidence enough that "talent" really exists.