The symptoms are:
$ ant -version
Unable to locate tools.jar. Expected to find it in /usr/lib/jvm/java-6-openjdk/lib/tools.jar
Apache Ant version 1.7.1 compiled on November 10 2008
$ java -version
java version "1.6.0_0"
OpenJDK Runtime Environment (IcedTea6 1.4.1) (6b14-1.4.1-0ubuntu10)
OpenJDK Server VM (build 14.0-b08, mixed mode)
The easiest way to solve this for me was to force Sun's “original” JDK.
Apparently, once you've installed
-jdkpackages, you have to
$ sudo update-java-alternatives -s java-6-sun
and then edit
/etc/jvmso that it looks something like this, with