EXECUTE AS clause (TSQL)

"By specifying the context in which the module is executed, you can control which user account the SQL Server 2005 Database Engine uses to validate permissions on objects that are referenced by the module." - BOL

The Options

{ EXEC | EXECUTE } AS { CALLER | SELF | OWNER | 'user_name' }

This applies to create procedure statement:

CREATE { PROC | PROCEDURE } [schema_name.] procedure_name [ ; number ] 
    [ { @parameter [ type_schema_name. ] data_type } 
        [ VARYING ] [ = default ] [ OUT | OUTPUT ]
    ] [ ,...n ] 
[ WITH <procedure_option> [ ,...n ] ]
[ FOR REPLICATION ] 
AS { <sql_statement> [;][ ...n ] | <method_specifier> }
[;]
<procedure_option> ::= 
    [ ENCRYPTION ]
    [ RECOMPILE ]
    [ EXECUTE_AS_Clause ]

SQL 2000->2005 migration

Breaking Changes to Database Engine Features in SQL Server 2005

Behavior Changes to Database Engine Features in SQL Server 2005