From Stroustrup's own homepage (right at the top):
The "special edition" is the hardcover version of the 3rd edition. It
differs from the early printings of the 3rd edition by about 1,000
corrections and clarifications, by two new appendices (just over 100
pages; also available online, see below), and by an improved index.
The only difference between the current printings of the special
edition and the 3rd edition is the cover (and the price difference
implied by that stronger cover).
(The bold emphasis is mine.)
A little more from his FAQ section:
Should I buy the "Special Edition" of "The C++ Programming Language"?
Sure! Buy it early and often :-)
Seriously, the difference between the current printings of the special
edition and the 3rd edition is just the hard cover (and the price
difference implied by that stronger cover).
If I were a C++ programmer who hadn't read The C++ Programming
Language (3rd Edition), I'd buy and read either the 3rd edition or the
special edition. If I used my textbooks and references heavily, I'd
choose the hard cover. The cover on the 3rd is the best soft cover
available, but it doesn't equal the special edition's hard cover.
If I already had the 3rd edition, I'd buy the SE if my current copy
were fraying or if my copy were an early printing (the 3rd now has
about 30 printings and SE about 20).
Compared to the first printing, the special edition and the most
recent printings of the 3rd edition have 1,000+ corrections and
clarifications. As a heavy C++ user, I find that significant. There
are also the two new appendices (just over 100 pages; available for
download: Locales and Standard-Library Exception Safety).
Existing material has not moved around so page numbers can be used to
refer to material in old printings, new printings of the 3rd edition,
and in the SE.
The SE also has an improved index.