I recommend _not_ putting JDBC drivers in jre/lib/ext for the following reasons:
(1) It makes it harder to deploy applications that might require different versions of the same driver (which is handled by the application-specific classloaders in your appserver)
(2) It can lead to strange errors, where you _think_ you've updated the JDBC driver in your application, however an old one is sitting around in jre/lib/ext and that one gets picked up first by the VM.
Once you learn how the CLASSPATH and classloaders in J(2)EE work, you will find that even though it's not as straightforward as dropping a .jar file in jre/lib/ext, it is a lot more flexible. Unfortunately if you put "infrastructure" libraries in jre/lib/ext you lose a lot of that flexibility.
-Mark
Mark Matthews
Consulting Member Technical Staff - MySQL Enterprise Tools
Oracle
http://www.mysql.com/products/enterprise/monitor.html