I just committed this to 0.18 only.
After temporarily removed src/test/org/apache/hadoop/dfs/TestDiskError.java, 0.18 with the patch can be compiled in Java 5. Tested locally, everything works fine.
NavigableMap is a new interface in Java 6. So for Java 5 its sub-interface SortedMap should be used.
Forgot to write a description when creating this issue. Fortunately, the title is already clear enough.
4823_20081210_0.18.patch: Use SortedMap instead of NavigableMap.