It seems as if parent's cache of children is not
updated on MasterFileObject.rename(). This is on
NetBeans 4.0 beta 2 (downloaded together with JDK
5.0).

To reproduce:

1. Create a new sample project "AnagramGame"
(ships with NB).
2. Invoke FileObject.rename() to rename 
"WordLibrary.java" to "NewName.java".
3. Ask the FO's parent for a child with the old
name (f.getParent().getFileObject("WordLibrary",
"java")). It must return "null" (no such file
exists anymore), but it returns the renamed file
instead ("NewName.java").

Here's a BeanShell transcript I used to verify
this (where variable "f" is the FileObject for
"WordLibrary.java"):

  bsh % print(f.name);
  
  WordLibrary
  
  bsh % print(f.getClass());
  
  class org.netbeans.modules.masterfs.MasterFileObject
  
  bsh % lock=f.lock();
  
  bsh % f.rename(lock, "NewName", "java");
  
  bsh % lock.releaseLock();
  
  bsh % print(f.name);
  
  NewName

  bsh %
print(f.parent.getFileObject("WordLibrary",
"java") == f);
  
  true