squall_web//5.0

Microsoft SQL Server starting with version 2000 supports clustered and non-clustered indexes being added to views.  They are referred to as Indexed Views.

Such cases to use one:

• Search Critera spans multiple tables
• Speeding up queries that reference a view inside them

References:

http://www.microsoft.com/technet/prodtechnol/sql/2005/impprfiv.mspx

General performance:

http://searchsqlserver.techtarget.com/tip/1,289483,sid87_gci1170220,00.html