I always like to test and re-test my predictions.
Rogers in the early 1950's on technology diffusion published an adoption curve for technology. Now, that curve is an average, but let's test my '50% of the market by January 2013' prediction versus a respected curve.
Since the X-axis (timeline) varies for every technology, I took two points from e-books, roughly where we are now and how long it took to achieve 2.5% market share. There are different measures and noise in the data, but it looks like end of April 2009 is my best estimate. (There is way too much noise in the data and AAP revises old data... so an exact time is tough without the raw data.)
That curve shows about 35% market for e-books in January 2013. :( 50% would be January 2014 per the Rogers curve. :(
Now I've already made my prediction public in numerous blog comments, so I'm stuck with the prediction. Technology adoption isn't a smooth curve... But perhaps I was a little optimistic. So we might see one third of the dollars in e-books at the start of 2013... But I still have a hunch it will be half.
Either way, I see tremendous market growth and huge opportunities for Indie authors.