I was having exactly the same problem, My servlet engine was mod jserv of apache.
Inspite of the mm.jdbc driver being in proper classpath i had this prob. Then what i did was just put the entire mm.jdbc drv into the servlets folder of the virtual host.
Then it somehow just started.
If i echoed classpath than it was showing the same in classpath even earlier,
Though this is not the right method to do it. But it works for me as of now, i have no logical reason to it though