From the Red Hat press releases web site: IDC Discusses the Health of the Linux Server Market. According to the IDC report:

“Spending on software related to Linux SOE [Server Operating Environment] platforms is growing quickly - with a 2006-2011 CAGR of 35.7%. Overall spending on Linux SOEs, including software, hardware, and services, is increasing at a 2006-2011 CAGR of 24.1%.”

The full report is available online.