I talked to Kazu about this and can tell you this:

We're having the current XOOPS version (2.0.5.1) tested to work with Postgress. This should work fine, but we're not 100% sure. At this moment, XOOPS can generate sql files for MS SQL server, postgresql, mysql, firebird, and interbase. These layers haven't all been implemented yet, but it's possible. We're also looking at ingres db integration.

The problem now, is that all SQL files need to be converted to the other database formats. For every module. This puts the brunt of the pressure on the module developers, and we can't expect them to know the db schema's of all the mentioned databases.

The next version of XOOPS (2.1) will make it a lot easier for module developers to support other db's: XOOPS will automatically generate the correct SQL files for the supported databases, based on an XML schema file provided by the module(developer) itself. Thus, it would be easy to add another db support to XOOPS, without changing all the modules

Herko