javac is /usr/bin/javac (at least it should be), and is installed with the Essentials package, so if it's not there, something weird is going on.
Note that /usr/bin/javac is simply a symlink, so also check for the target, which is /System/Library/Frameworks/JavaVM.framework/Commands/javac (which should have been updated by the Java 1.3.1 update).