/usr/lib/jvm has:

    lrwxrwxrwx 1 root root 17 May 22 23:44 icedtea6 -> /usr/lib/icedtea6
    lrwxrwxrwx 1 root root 21 May 22 21:29 sun-jdk-1.6 -> /opt/sun-jdk-1.6.0.20

"eselect java-vm list" reports:

    Available Java Virtual Machines:
      [1]   icedtea6 
      [2]   sun-jdk-1.6  system-vm

"eselect java-vm set system 1" runs without complaint, after which "eselect java-vm list" reports:

    Available Java Virtual Machines:
      [1]   icedtea6  system-vm
      [2]   sun-jdk-1.6 

"eix -e --nocolor ant-core" reports:

    [I] dev-java/ant-core
	 Available versions:  1.7.1-r4{tbz2} ~1.7.1-r5 ~1.8.0-r1 ~1.8.0-r2 ~1.8.1 {doc elibc_FreeBSD source}
	 Installed versions:  1.7.1-r4{tbz2}(08:02:48 AM 03/08/2010)(-doc -elibc_FreeBSD -source)
	 Homepage:            http://ant.apache.org/
	 Description:         Java-based build tool similar to 'make' that uses XML configuration files.
    
"emerge -qv ant-junit" fails because ant-junit isn't found

 * CPV:  dev-java/ant-junit-1.7.1
 * REPO: gentoo
 * USE:  amd64 elibc_glibc kernel_linux multilib userland_GNU
!!! ERROR: Package ant-junit was not found!
 * Unable to determine VM for building from dependencies:
NV_DEPEND: dev-java/junit:0  >=dev-java/java-config-2.1.9-r1 >=sys-apps/portage-2.1.2.7 
	
	>=sys-apps/portage-2.1.2.7
	>=dev-java/javatoolkit-0.3.0-r2 >=virtual/jdk-1.4
	>=virtual/jre-1.4
	~dev-java/ant-core-1.7.1
		!dev-java/ant-tasks

root@osage tmp # eselect java-vm set system 2

root@osage tmp # eselect java-vm list
Available Java Virtual Machines:
  [1]   icedtea6 
  [2]   sun-jdk-1.6  system-vm

!!! Warning: VMs marked as Build Only may contain Security Vulnerabilities and/or be EOL.
!!! Warning: Gentoo recommends not setting these VMs as either your System or User VM.
!!! Warning: Please see http://www.gentoo.org/doc/en/java.xml#build-only for more information.

root@osage tmp # emerge -qv ant-junit

 * CPV:  dev-java/ant-junit-1.7.1
 * REPO: gentoo
 * USE:  amd64 elibc_glibc kernel_linux multilib userland_GNU
!!! ERROR: Package ant-junit was not found!
 * Unable to determine VM for building from dependencies:
NV_DEPEND: dev-java/junit:0  >=dev-java/java-config-2.1.9-r1 >=sys-apps/portage-2.1.2.7 
	
	>=sys-apps/portage-2.1.2.7
	>=dev-java/javatoolkit-0.3.0-r2 >=virtual/jdk-1.4
	>=virtual/jre-1.4
	~dev-java/ant-core-1.7.1
		!dev-java/ant-tasks
 * ERROR: dev-java/ant-junit-1.7.1 failed:
 *   Failed to determine VM for building.
 * 
 * Call stack:
 *             ebuild.sh, line   48:  Called pkg_setup
 *             ebuild.sh, line 1346:  Called java-pkg-2_pkg_setup
 *     java-pkg-2.eclass, line   63:  Called java-pkg_init
 *   java-utils-2.eclass, line 2126:  Called java-pkg_switch-vm
 *   java-utils-2.eclass, line 2550:  Called die
 * The specific snippet of code:
 *   				die "Failed to determine VM for building."
 * 
 * If you need support, post the output of 'emerge --info =dev-java/ant-junit-1.7.1',
 * the complete build log and the output of 'emerge -pqv =dev-java/ant-junit-1.7.1'.
!!! When you file a bug report, please include the following information:
GENTOO_VM=  CLASSPATH="" JAVA_HOME=""
JAVACFLAGS="" COMPILER=""
and of course, the output of emerge --info
 * The complete build log is located at '/var/log/portage/dev-java:ant-junit-1.7.1:20100523-125743.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-java/ant-junit-1.7.1/temp/die.env'.
 * S: '/var/tmp/portage/dev-java/ant-junit-1.7.1/work/apache-ant-1.7.1'

 * IMPORTANT: 1 news items need reading for repository 'gentoo'.
 * Use eselect news to read news items.