One thing already mentioned in the other posts here, but that I would like to emphasize more, is the fact that ArcGIS for Desktop and ArcGIS for Server are almost completely database platform independed as regards to the user functionality offered from within the software packages. This is one of the major achievements of ESRI regarding database support in GIS, and often under-appreciated or not well understood by people less familiar with ESRI software.
Essentially, you can simply swap out any database platform and replace it with another, e.g. throw out Oracle and install Microsoft SQL Server. Or throw out SQL Server and replace it with PostGIS, DB2 or any of the other supported platforms.
You can even "mix" database platforms. Replicate from an Oracle to a SQL Server database, or even to a "simple" File Geodatabase...
ArcGIS for Desktop or Server simply don't care, you can keep doing what you did yesterday on the old platform if you make a switch! (and that is a great thing). ESRI really is on its own with such diverse database platform support, yet unified user experience.
The only "but" is that you need to stay within the ESRI Geodatabase context... Start messing at the database level, and of course you will need to be fully aware of the multitude of differences between database platforms, even if all of them are supposed to "talk" "standard" SQL.
