http://www.macnewsworld.com/story/Vs4UF3ipCUcBwU/High-End-Accessories-Essential-to-iPod-Experience.xhtml
"High-End Accessories Essential to iPod Experience"

Complements, Cooperation, and Competition
commentary by Mark Pearson

This article describes a market of complements around a product, in this
case the iPod.  While the article isn't very deep, it does implicitly pose
some interesting questions (only some of which I'll attempt to answer
here).  For instance, of the approximately five hundred iPod accessories,
Apple only provides twenty-three.  What is the strategic decision behind
Apple not being more active in the market of complements around its own
product?

Apple has an incentive to keep the market of complements vibrant and 
competitive.  After all, for those people that consider certain 
accessories necessities, the cheaper the accessory, the more Apple can 
charge them for the iPod itself.

Because of the strength of Apple's brand, competitors would be leery of
offering accessories that duplicate Apple's offerings, as customers would
probably buy _Apple_ accessories (when they are available) to go with the
_Apple_ iPod.  Hence the accessories markets in which Apple participates
are likely to be less competitive.  In turn, if Apple enters many
accessories markets, competitors might expect Apple to enter more and
therefore not even bother trying to offer iPod accessories at all.  This 
seriously harms the innovativeness and competitiveness of the accessories 
market.

This is also an example of businesses with whom Apple simultaneously
competes and cooperates: Apple needs to give out information on iPod 
specifications to others to let them build accessories while it is 
simultaneously competing with some of these businesses in a few of the 
accessories markets.  How Apple chooses to handle this might obviously 
depend on the information needed for the accessory, how revealing it 
is about Apple's (trade) secrets, how important Apple believes having the 
particular accessory available to consumers is to its own business, and, 
likely, whether Apple could be liable for anti-competitive practices by 
holding back certain information.  (Apple does indeed have a very powerful 
position in the handheld music player market, with the majority of the 
market share.)

As a final tangential thought, the iPod is a fashion statement and many
accessories (e.g. covers) are to the iPod what fashion accessories are
to the original piece of clothing.  I wonder if Apple has a few
consultants previously employed in the clothing fashion business to see
what insights they have on how accessories and the accessories market
effect business development.  (They probably do.  I wonder what they say.)