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I think that selling more eBooks than hardcopies is a good enough benchmark.
Personally, I think that Amazon alone is enough. But I would be willing to push the target a little bit back and include B&N's and Borders' websites as well.
I might be willing to consider pushing the target back even further and include B&N's and Borders' b&m stores. For example, when the day comes that B&N as a company sells more eBooks than hardcopies (by whatever means), then the eBook would clearly be mainstream.
I don't think that the percentage of the population or the percentage of readers (people who read) is as important as the percentage of books.
By the way, for purposes of this discussion, I would include free downloads of public domain works. Free eBooks might not help the retailers and the publishers, but the idea of "mainstream" concerns what the public is doing, not how profitable the businesses are.
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