That link from Rob really helped me out. Thanks.
I'd like to add that it's fixed from at least JDK1.6.0_16 (but perhaps some version before). So you can use the newest available JDK and it should work.
I had this problem because in my Eclipse's
Ant preferences I had a reference to an older version (JDK1.6.0_05) tools.jar under Global Entries.