SQL Server 2008 will include GROUPING SETS; a (very) short explanation is that these allow the equivalent of multiple GROUP BY clauses in a single SQL statement. The result is a UNION ALL of the resultsets. SQL Server 2008 also contains/allows standard syntax for ROLLUP and CUBE, which have been in SQL Server for a while.
One way to use (or to think of) grouping sets is that, while ROLLUP with N columns produces a UNION of N+1 results and CUBE produces N-squared -1 results, grouping sets can produce an intermediate number of results, when not all the dimensions produced by CUBE are needed. Grouping sets should allow better optimization of this type of dimensional query.
SQL Server 2008 Reporting Services will contain a new type of control, called the TABLIX. The SQL Server 2008 CTP BOL defines a TABLIX as: "A Reporting Services RDL data region that contains rows and columns resembling a table or matrix, possibly sharing characteristics of both." Grouping sets sound like a good fit with this component. When TABLIX is available, a short profiler session would confirm this.
Of course, in addition to performance benefits and TABLIX support, GROUPING SETS are part of the ISO-ANSI SQL-2006 spec. Another plus.