Below are some useful resources on the topic of archiving relational databases. Some refer to specific methods and software tools, others are general papers on the topic.
Starting with two similar European projects which have developed an XML format for archiving relational databases (and by association spreadsheets) along with open source software tools for the conversion of databases to this format:
..and for something completely different
D2R Server is a tool for publishing relational databases on the Semantic Web. It enables RDF and HTML browsers to navigate the content of the database, and allows applications to query the database using the SPARQL query language. Is this an option for converting relational databases into a self-describing RDF triple-store which can be used as a long-term preservation format ?
Some general papers on the topic: