I've been talking up some evil plans I have for Drupal's database system in Drupal 7 lately, without going into a great deal of detail. For the most part, I've been trying to avoid distracting people, myself included, from the considerable work still remaining on Drupal 6. However, there has been recent work and discussion of making post-freeze changes to Drupal 6's database system, and even backporting them to Drupal 5. Those changes revolve mostly around database replication, which Drupal currently doesn't support in any meaningful way. That becomes important, though, on very heavy sites like, say, I don't know, Drupal.org. :-)
Those changes, though, impact the API changes we can make in Drupal 7. (OK technically they don't, since we change APIs all the time, but if we can set things up so as to minimize API changes over time that does make life easier for everyone.) For that reason, at Dries suggestion, I'm going to try and lay out now a skeleton of what I'm planning for Drupal 7's database system and how we can start adding replication support to Drupal 6 in a way that flows into it.