I've looked at Visual Studio release patterns. It seems that Microsoft has always cared about the extended support final date, until now.

VS2008 - November 2007. Drops support for Windows 98 (mainstream support January 2004, extended support July 2006). Windows 98 market share when VS2008 was released: 1.0%.

VS2010 - April 2010. Drops support for Windows 2000 (mainstream support June 2005, extended support July 2010). Windows 2000 market share when VS2010 was released: 0.5%.

VS11 - Beta February 2012, RTM 2012. Attempts to drop support for Windows XP (mainstream support April 2009, extended support April 2014). Windows XP market share when VS11 Beta was released: 30.0%. Probabilistic calculation says it'll be 24% by the end of 2012.

This clearly shows that for the first time Microsoft is trying to drop Visual Studio support for an OS that still has extended support (let's ignore Windows 2000, the gap was only a few months).

NOTE: These market shares are given if linux, mac and mobile platforms are also considered. When considering Windows machines only, Windows XP had a market share of 36% in February 2012, by the end of 2012 it'll be 29%.