Hooray! The MySQL DBD driver is finally (probably for some time now) included in the debian distribution of Apache2. :-)
I've been waiting for this for years, believe it or not. I'm quite pleased because I mainly use MySQL as a database and digest authentication for HTTP Auth, and now I can use them both together.
The original problem was caused by licensing, but the MySQL folks figured it was worthwhile to make an exception for Apache. Good choice.