The dev-java/plexus-utils-1.5.1 ebuild refers to "http://dev.gentooexperimental.org/~kiorky/${P}.tar.bz2" The closest verion I could find there was "http://dev.gentooexperimental.org/~kiorky/plexus-utils-1.4.7_pre20071021.tar.bz2", not .../plexus-utils-1.5.1.tar.bz2 The plexus-utils-1.5.1.tar.bz2 file does not appear to be in the gentoo mirror either.
I ran in to this problem too. The ebuild should really be removedi or at least masked if the source isn't in any mirrors.
Ebuild removed from overlay.
Seemingly still maintained but it's doubtful whether we'll ever build Maven from source again.