Unique index on a Partitioned table

Partition columns for a unique index must be a subset of the index key >

ALTER TABLE Data.Event

ADD CONSTRAINT UniqueReference

UNIQUE(EventDateTime,EventRef)

http://databases.about.com/od/sqlserver/a/unique.htm
http://www.simple-talk.com/sql/database-administration/partitioned-tables-in-sql-server-2005/
http://msdn.microsoft.com/en-us/library/ms190273.aspx