The move is bound to invite criticism from some quarters, as many GBA SP owners have already had to pay extra for a headphone adapter for the console, and this will undoubtedly be seen as yet another feature which Nintendo should have included with the console from the outset.
However, for Nintendo's part, the move is a shrewd one. Companies have been making significant amounts of money by marketing optional accessories for the GBA for quite some time, and now Nintendo apparently plans to do likewise. It's thought that it already makes a significant profit on GBA SP hardware sales, with the £89.99 console costing as little as £20 to manufacture according to some estimates.
Source: The Magic Box