A colleague of mine (thanks Jimbo!) pointed me towards a benchmark comparison of Postgres 8.3.8 on RHEL 5.4 versus SQL Server 2008 R2 on Windows Server 2008 R2, performed and written up by Red Hat. I hadn't seen it before, so figured that maybe others hadn't either:
http://www.redhat.com/pdf/rhel/bmsql-postgres-sqlsrvr-v1.0-1.pdf
Can't wait for the result? The elephant wins :-)