What is ESRI Portland using PostGIS for? A replacement to SQL Server, perhaps? Are they still using ArcSDE on top of PostGIS?
Our organization is tied to SQL Server, and we run ArcSDE on top of that. We also, however run a SQL-only database, with native geometry loaded via FME / Data Interop. Extension. Most of the student-level analysis we do is accomplished using SQL Spatial Functions (ST_Intersect, ST_Distance, ST_Buffer, etc.).
In our case, would PostGIS replace SQL Server? Would we still load ArcSDE on top of it and use PostGIS as only the 'geometry-enabling' back-end, using ArcMap/Catalog as the front-end?
It's also important to keep the technology stack straight. In your situation, PostGIS is not a replacement for SQL-Server, PostgreSQL would be. PostGIS would be a replacement for the GEOMETRY/GEOGRAPHY native to Microsoft's database (or for the SDE.ST_GEOMETRY created when an enterprise geodatabase is enabled). ArcSDE doesn't go "on top of" databases anymore, it's just the access technology framework for enterprise geodatabases (and as such, can't be bypassed in a geodatabase; only Query Layers can bypass ArcSDE technology, but then you don't get versioned editing).
One thing to remember is that the Portland Esri office is an R&D center, and thus it is part of our jobs to explore all aspects of GIS technology, both open and closed source.
It's also important to keep the technology stack straight. In your situation, PostGIS is not a replacement for SQL-Server, PostgreSQL would be. PostGIS would be a replacement for the GEOMETRY/GEOGRAPHY native to Microsoft's database (or for the SDE.ST_GEOMETRY created when an enterprise geodatabase is enabled). ArcSDE doesn't go "on top of" databases anymore, it's just the access technology framework for enterprise geodatabases (and as such, can't be bypassed in a geodatabase; only Query Layers can bypass ArcSDE technology, but then you don't get versioned editing).
