wp-db.php is now always included. Database drop-ins should continue to work without any adjustment in most cases (or can be adjusted to work with small tweaks).
This is for performance as we are no longer conditionally including a file. It also allows db.php files to cleanly extend wpdb, which previously required a bit of tinkering.
Ben, 
Aaron Brazell 7:24 pm on September 20, 2010 Permalink
So, essentially, a db.php class extends and instantiates itself?
Correct?
Ryan Boren 8:26 pm on September 20, 2010 Permalink
Correct. Hyperdb provides an example of how to do this in a way that is compat with 3.0 and 3.1.
Ben 2:29 pm on September 27, 2010 Permalink
What about finally fixing the problem with inserting null values.. ?
this is a really annoying bug for plugin developers..
(http://wordpress.org/support/topic/bug-fix-wpdb-insert-amp-update-with-null-values)
Peter Westwood 8:48 pm on October 19, 2010 Permalink
The best way to report bugs like this is in trac – https://core.trac.wordpress.org/
I’ve created a ticket here: #15158
Now we just need a patch for the improvements