So you want to have a full-fledged low-footprint database on your mobile phone while being able to replicate this data (even bidirectionally) to any relational database (Oracle, MS-SQL, MySQL, PostgreSQL, DB2, etc.) or db4o? Your wait is over!
You won't believe how easy it is to set up a full fledged mobile to RDBMS replication with a cooler database on the mobile device than SQLite.
In the authors' own words: "We believe that db4o is the simplest and most powerful solution. What alternatives are out there? First of all you need a mobile phone that is able to run a database. If you choose Android you could use SQLite, send the data with any protocol (REST, SOAP?) to a server and store it in your relational database. Nevertheless we think db4o is far easier to develop while using OO languages as C# or Java, it has lots of features that SQLLite doesn't offer and the way to deal with the database is very easy"
Check the mdRS project page for more info and source code. Thanks Stefan & Roman!