The 1.5.0 javadoc for java.lang.System states:

  The current set of system properties for use by the getProperty(String)
  method is returned as a Properties object. If there is no current set
  of system properties, a set of system properties is first created and
  initialized. This set of system properties always includes values for
  the following keys: ...

And among the keys listed is java.compiler. The above statement thus
quite clearly claims that a value for java.compiler always should
be included.

However, when executing a small program that lists the system
properties (see attachment), java.compiler is lacking when using
1.5.0. Further, the 1.5.0 javadoc for java.lang.Compiler states:

  When the Java Virtual Machine first starts, it determines if the system
  property java.compiler exists. (System properties are accessible through
  getProperty and , a method defined by the System class.) If so, it is
  assumed to be the name of a library ...

I interpret this as "sometimes the system property java.compiler could
be missing".

So, in addition to aforementioned discrepancy between implementation and
documentation, there also appears to be a discrepancy (contradiction)
within the documentation itself.