Dave Bouwman has some thoughts on using PostgreSQL as a RDBMS for ArcSDE (or ArcGIS Server Enterprise as we should be calling it).

Thus far I’ve simply come to realize that I have a lot to learn. I need to grok a lot more about Postgresql and PostGIS to start, and then add ArcSDE into the mix.

While everyone is really excited about PostgreSQL support at 9.3, remember it won’t be as easy to administer as SQL Server is (at least to those who already use SQL Server). Just keep that in mind before abandoning SQL Server outright for PostgreSQL.