Changeset http://fisheye4.atlassian.com/changelog/hudson?cs=7102
The fix uses deleteOnExit, there was a problem with deleteOnExit of tmp-files in older Java-releases, I
don't know wether this is fixed:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4513817
If I understand it correctly every build will produce a tmp-file, the list will grow rather rapidly.

http://fisheye4.atlassian.com/browse/hudson/trunk/hudson/main/core/src/main/java/hudson/Main.java?r=21479#l113 shows that deleteOnExit is still there, so propably it is still a problem.

Another bugs refering to this with a solution for JAVA 1.6 is:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4872014
which references:
http://www.jroller.com/javabean/entry/solving_an_outofmemoryerror_java_6

The best solution propably would be to use a try...finally block to get rid of the temporary file.