When I was a regular on the old Compuserve Science Forum, the I.Q. debate came up fairly frequently, and one of the standard tangents was to start talking about sports and athletics. It is, after all, easier to see the consequences of athletic ability, and there are many, many statistics that are generated in sports as a matter of course, without having to create special tests to gather them.
One of the participants in one discussion reported that there was a considerable literature attempting to attribute success in sports to some separately testable ability, or even to correlate cross-sport success, to account for Bo Jackson, for example, although the case of Michael Jordan was also mentioned.
At any rate, our correspondent reported that all such statistical studies of sports yielded one primary component that correlated with success: the amount of time spent practicing and playing the sport in question. Everything else just vanished into the noise.
Now the "talent" side of the talent vs practice argument can come up with a quick explanation for the practice effect: self-selection. People who are naturally gifted at a given sport tend to enjoy playing and practicing that sport, so they practice more, etc. The difficulty with that explanation is that it is devilishly difficult to predict who is "talented" at a particular sport a priori, at least insofar as creating a testing matrix that will give a good prediction of later success.
There is a similar argument that has recently been made by the authors of Freakonomics.
Dubner and Levitt cite data that suggests that children who are some months older when they begin to play soccer (football to you Euros), owing to what time of the year the children were born, tend to be more heavily represented in professional teams. The argument here is that those few extra months of growth make the children a bit larger and quicker (on average) when they begin to play, so they have an extra edge, tend to do better from the beginning, and, hey, who doesn't spend more time on something that they're better at?
Notice, however, that we're talking about some pretty slim differences in ability here, equivalent to a few months worth of extra maturity, with the differences diminishing over time. So what you're really getting is a very strong feedback loop, where a little extra preliminary edge translates into huge differences in training and practice over time.
The Freakonomics source for this material is K. Anders Ericsson, who was probably the source for my original correspondent on Compuserve (thus do loops close on the Internet). Ericsson has been studying the practice effect phenomenon for several decades, and he's quite convinced that there is much less to talent than commonly believed. He does note that there are some sports that have physical constraints (no one has ever had to make the choice between being a Sumo wrestler or a jockey, for example), but other than that, Ericsson seems to believe that it's all about practice, and, more importantly, effective practice.
Again, drawing from sports, records are still being broken in most sports, on a regular basis. At some point, the total time spent on practice fills all available time, so there must be some improvements in how athletes practice in order for improvements to continue. And even if the improvements are such things as the use of anabolic steroids and HGH, there are still more (and less) effective ways of using those things. Weight training without steroids is more effective than the other way around, for example.
There's one other phenomenon that is often overlooked in the talent/practice debate, however, and that is the random factor. In sports, that is most obvious in how injuries can derail careers. Certainly some of the improvements in training over the past century involve learning how to train with less risk of being injured, but in the heat of the game, injuries happen. Then, later, great pressure is placed on the athlete to "play through it," and thereby increase the severity of the injury. It takes a really tough-minded coach to demand that a player wait until fully recovered before resuming competition. Most are going to go for the appearance of tough-mindedness; after all, there are always plenty more guys who want to play the game. Besides, it's hard to keep up the practice from the sidelines.