^ The problem may have been the W220 at the time. Some folks may have switched to the R230 instead. Should MB drop the SL in the next generation, and have the S-Class cabrio instead? 
Some MBUSA sales numbers...
2001 ....... 4,217
2002 ...... 13,717
2003 ...... 13,318
2004 ...... 12,885
2005 ...... 10,080
2006 ....... 8,462
2007 ....... 6,126
2008 ....... 5,464
2009 ....... 4,025
2010 ....... 2,385
2011 ....... 1,449
2012 ....... 4,899 <-- The new R231 SL debuts in June 2012 in the USA.
2013 ....... 7,007
2014 ....... 2,542 <-- First half
http://en.wikipedia.org/wiki/Mercedes-Benz_SL-Class

Some MBUSA sales numbers...
2001 ....... 4,217
2002 ...... 13,717
2003 ...... 13,318
2004 ...... 12,885
2005 ...... 10,080
2006 ....... 8,462
2007 ....... 6,126
2008 ....... 5,464
2009 ....... 4,025
2010 ....... 2,385
2011 ....... 1,449
2012 ....... 4,899 <-- The new R231 SL debuts in June 2012 in the USA.
2013 ....... 7,007
2014 ....... 2,542 <-- First half
http://en.wikipedia.org/wiki/Mercedes-Benz_SL-Class