With the new PDO-based database API now in core (yay!), there are a number of follow-up tasks that need to be taken care of. They were left out of the original patch for a variety of reasons, mostly keeping the already massive patch from becoming even more massive. :-)
The full Drupal 7 database issue queue contains over 50 items that should be addressed at some point, but the following in particular relate directly to DBTNG. Volunteers welcome and encouraged to help out!
- http://drupal.org/node/299308 - Explicitly require PDO in the installer. We do kinda require this now.
http://drupal.org/node/298669 - First steps toward restoring database query logging in order to make devel.module work.- http://drupal.org/node/67349 - Support SQLite databases. Really. We want this. A lot. :-)
- http://drupal.org/node/195416 - Make database table prefixes per-connection. If you want to use prefixing and multiple database connections, well, right now you can't. Let's make it possible. This does impact SimpleTest, too, but if done properly can probably make simpletest's code easier.
- http://drupal.org/node/111011 - Add foreign key support to Schema API. Even though core can't leverage them directly, extra metadata can be useful for PHP-level maintenance. It also is a prerequisite for letting Views leverage Schema API directly.
- http://drupal.org/node/299268 - Support "virtual fields" in Schema API.
- http://drupal.org/node/299176 - Exterminate db_rewrite_sql(). And there will be much rejoicing.
- http://drupal.org/node/299178 - Support subqueries in FROM clauses.
- http://drupal.org/node/299267 - Add "extenders" to Select queries to support tablesort, pager, and views.
- http://drupal.org/node/299269 - Flag slave-server-safe queries for better SQL replication.
- http://drupal.org/node/299433 - Figure out how we want to remap dynamic query operators.
And of course, convert all of core to the new API so we can remove the BC layer. :-) That can and should happen in bits and pieces, as well as part of any new patches from now on.