SQL Server 2005 introduced only partial support for the OVER clause. It is our strong believe that a more complete implementation of the OVER clause should be prioritized highly in consideration for future enhancements in SQL Server. A more complete implementation of the OVER clause can help in solving many common business problems with simpler, more intuitive, and faster solutions than available today and also substantially reduce the need for cursors.
The following paper details the feature enhancement requests:
The current feedback item addresses the OVER clause enhancement request - LAG and LEAD functions, discussed in the paper in section 4 item ii.