Today I got to work and discovered that my ant build for our J2ME app is failing randomly. After a long debugging sequence, I discovered that some Java 1.6 update has installed java.exe in c:\windows\system32. This particular version of java.exe is for the JRE and sets java.home to the JRE home directory. Since that path is high in priority in my system path, eclipse took it as its runtime environment, resulting in me having a neutered JRE (no JDK tools). Thus, jar.exe was missing from my java home and nothing would build.
Sun, if you are reading this, please don’t try to succumb to the crappy Windows way of doing things.