I'm glad to see people start to understand the benefits of expanding mapping technologies to non-relational data sources: seen on TSS, Java Persistence API (JPA) implementation for Amazon SimpleDB.
Xcalia has been a pioneer in this area with Castor (an ORM-OXM open source product). Then later, Toplink announced support for XML and JCA data sources, while JPOX now supports db4o.
This industry is going in the right direction. That said the real challenge is when you want to address service-oriented data sources. In this area Xcalia is still ahead of the competition.