One of the sessions at DrupalCon I attended was Larry Garfield's talk about "Drupal Databases: The Next Generation", which gave me a good insight into the current state of the Drupal database layer and how they plan to overhaul it for Drupal 7. The key points that I took away:

• A new API based on PDO
• Object-oriented, requiring PHP5
• Support for using prepared statements
• A unified access API
• A query builder
• More support for other database systems (currently Drupal supports MySQL and PostgreSQL only). In particular, they are keen on adding SQLite support, to ease local development.
• Support for master-slave replication (by randomly distributing reads among the hosts)
• Support for using different database types in parallel (e.g. using SQLite for read-only tables, MySQL for everything else)

The slides and a video of the presentation are available, if you want to check it out. There is a task list on the Drupal.org web site that keeps track of the ongoing activities.