In last couple of years, I have seen many medium to large scale businesses have started looking into SQL Server as an option to replace their existing Oracle databases.
Here are some intresting links to research and studies showing the growth rate of SQL Server 2005/2008 in RDBMS market.
SQL Server is the fastest growing Database and Business Intelligence vendor
SQL Server ships more units than Oracle and IBM combined.
SQL Server is the #1 OLAP Server on the market.
SQL Server is more secure than Oracle. Since July 2003 more than 100 critical Oracle database security vulnerabilities have been identified, compared to ZERO for SQL Server for that period.
SQL Server is best in price/performance for TPC-H 1TB & 3TB (non-clustered).
Microsoft SQL Server and Unisys are the first to publish TPC-E benchmark results.
Gartner 2006 worldwide RDBMS market share reports highest growth rate in market share for Microsoft SQL Server. With 28% growth in market share, growth is almost twice the nearest competitor at 14.8% growth.