The /usr/bin/ant script from ant 1.5 (and 1.5.1) resets the $CLASSPATH variable without regard to the $CLASSPATH set in the environment. Obviously, the script should respect the $CLASSPATH which has been set by the user. Fixing this (for both 1.5 and 1.5.1) should be trivial: Just don't override the environment $CLASSPATH if it has been set by the user. (And yes, I am aware that one can create a ~/.gentoo/java-env containing a CLASSPATH, but that is no reason to ignore $CLASSPATH in the environment. ~/.gentoo/java-env is not exactly useful unless *all* your development machines are Gentoo. Additionally, setting the environment is much easier to do dynamically).
Fixed in portage tree as ant-1.5.1-r2 marked as testing.
