747b0392__5_1___Postgres_OnLine_Journal__Microsoft_SQL_Server_2008
[Feature] Partial Indexes - e.g. you want to create a unique index but only consider non-null values [MySQL 5.1] No [PostgreSQL 8.4] Yes [Microsoft SQL Server 2008] 
Yes - as of SQL server 2008 See Tom's notes and called Filtered Indexes.
Cross Compare of PostgreSQL 8.4, SQL Server 2008, MySQL 5.1 - Postgres OnLine Journal
Microsoft SQL Server 2008