I wanted to clarify one issue here, explicitly.
Discoverer and the Oracle Database 10g Express Edition (aka XE) are not supported. I realize that it may appear to some people that because Discoverer relational works on XE (with the exception of some functionality) and because I, as an Oracle BI PM have been posting about this to the whole world, that there is some official sanction to this. That is not the case.
Discoverer and XE are not supported.