It's hard to get a clear picture on the magnitude and speed of ebook growth. Perhaps matching the OP's Publisher's Weekly article with one from PW in February regarding
the same 13 publishers will shed light, as it is oranges vs oranges:
Ebook Growth in all of 2009:
176%
Ebook Growth in just the first quarter of 2010:
252%
Whatever the total percent of book sales they may be, it sure looks to me that ebook sales are accelerating dramatically. This is before the iPad began bringing ebooks into the awareness of the general public.
The next year or two should be interesting.