Hi Petr,
At the moment it's not that easy to integrate a XML provider in to Umbraco. Ruben V. did a great job in creating the datalayer so Umbraco can run on MySQL. But I would like to see even more abstraction by making use of the provider pattern. DotNetNuke does this and probably also this blogengine.
Now, if you want to run Umbraco on Oracle you have to build it in to the source.
With the provider pattern you could make your own project and inherit from a abstract class in the core.
This way Umbraco get's even more extensible. You could use this for membership, logging, data access, ....
Don't know how the core team thinks about this approach.
Converting a DotNetNuke site to Umbraco :
Follow it here