Yes, the upgrade changed the mysql root password, but not much else. This shouldn't cause a problem, as you shouldn't be using the root user for much of anything anyway.
That said, I'm not at all sure that upgrading to 5.1.2 will do you any good. The PHP included with 4.1.2, AFAIK, includes all the necessary functions. FWIW, though, that exact syntax works for me on 5.1.2 with my PHP upgrades installed.