Initially suggested by jglick when I asked a question on the developer group https://groups.google.com/forum/?fromgroups=#!topic/jenkinsci-dev/H8gAZcDOqgc -

Creates a delegating ExecutorService implementation whose submit and related methods capture the current SecurityContext
and then wrap the task in a block that resets the context afterwards. Using this one could simply write:

SomeUtilityClass.wrapExecutorWithSecurity(service).submit(new Callable<Void>() { 
     public Void call() throws Exception { 
         Jenkins.getInstance().getItem(whatever); // should work 
         return null; 
     } 
});