Financially, Apple has to decide if they want to continue to loose money and continue to give out free bumpers, or if the media frenzy has calmed down enough that they can continue to charge $29 for each bumper. September 30th is the end of the 2010 Fiscal Year and the End of the Financial Quarter. They are not redesigning the iPhone 4's antenna.
United States
The U.S. government's fiscal year begins on October 1 of the previous calendar year and ends on September 30 of the year with which it is numbered. Prior to 1976, the fiscal year began on July 1 and ended on June 30. The Congressional Budget and Impoundment Control Act of 1974 stipulated the change to allow Congress more time to arrive at a budget each year, and provided for what is known as the "transitional quarter" from July 1, 1976 to September 30, 1976.
As stated above, the tax year for a business is governed by the fiscal year it chooses.
For example, the United States government fiscal year for 2010 ("FY 2010" or "FY10") is as follows:
1st Quarter: October 1, 2009 - December 31, 2009
2nd Quarter: January 1, 2010 - March 31, 2010
3rd Quarter: April 1, 2010 - June 30, 2010
4th Quarter: July 1, 2010 -
September 30, 2010
http://en.wikipedia.org/wiki/Fiscal_year