The Apple fiscal year
Apple does not use a calendar year to segregate quarters rather, Apple's fiscal year is divided as follows:

1. First Quarter (Q1) - October, November, December
2. Second Quarter (Q2) - January, Febuary, March
3. Third Quarter (Q3) - April, May, June
4. Fourth Quarter (Q4) - July, August, September

The table below details total Mac shipments over the past thirteen fiscal quarters in thousands of units.  For example, 2611 units indicates 2.611 million units.  The Year/Year Change highlights the percent change in shipments over the comparable quarter from the prior year.  For example, Q4 2008 is compared to Q4 2007 and Q1 2009 is compared to Q1 2008.

Q1 2009 analysis
As shown, Q1 2009 experienced an 8.84 percent year-over-year growth versus Q1 2008, a 35.6 percent deceleration versus the 44.40 percent growth Q1 2008 experienced over Q1 2007.  This growth slowdown occurred during the busiest consumer buying period of the year and during a time of global economic contraction.  Note that Apple released updates to the entire portable line, the MacBook, MacBook Pro, and MacBook Air on October 14, 2008.  Portable shipments hit a new record of 1.796 million units, an increase of 33.83 percent over the year ago quarter.  Desktops on the other hand, declined 25.48 percent over the same period.

Q4 2008 analysis
Q4 2008 experienced 20.66 percent year-over-year growth versus Q4 2007, a 13.8 percent deceleration versus the 34.41 percent growth that Q4 2007 experienced over Q4 2006.  That slowdown occurred despite Apple conducting their most  aggressive back-to-school Mac promotion in the firm's history.  The data clearly shows that Apple was unable to exceed their growth performance from the prior year.

The growth deceleration during Q4 2008 was the first time since Q4 2006 that quarterly Mac shipments have experienced deceleration on a year-over-year basis.  The Q4 2008 to Q1 2009 data mark the first interval during which Apple has experienced two consecutive quarters of Mac shipment deceleration, dating back to Q3 and Q4 2006.

Mac growth deceleration to continue?
Mac shipments are increasing but at a slower rate.  This can be easily seen via the quarterly and aggregate yearly totals.  Not to be lost in the numbers however is the 37.78 percent increase in 2008 Mac shipments versus 2007.  This exceeds the 32.96 percent 2007 experienced over 2006.

On a more granular level, the Q1 2009 and Q4 2008 metrics demonstrate that despite shipping more Macs versus prior comparable quarters, Apple is experiencing slower Mac growth.  Therefore, the issue at hand is if this deceleration trend will continue into future quarters.  The chart below tracks Mac desktop and portable shipments over the past thirteen fiscal quarters.

The dramatic drop in desktop shipments was the the culprit for the Mac growth slowdown during Q1 2009 having dropped 25.48 percent from the year ago quarter and 22.22 percent from the prior quarter.  Desktop shipments have fallen for two consecutive quarters and three out of the past four quarters.  Clearly, portables are driving Mac sales and picking up the slack.  Perhaps the rapid decline in desktop shipments can be attributed to a lack of updates to the desktop lineup.  The iMac was last updated in April 2008, the Mac Pro in January 2008, and the Mac mini in August 2007.  Once Apple releases updates to the desktop models, we'll gain insight into buyer intentions and willingness to buy.