Short version: 
this is expected and the error message clearly says why:
only variables can be referenced and you cannot make a reference to an expression result.
Long version:
In your case f_default() returns *copy*, not a reference as you wrongly consider, while f_ref() returns reference and should work indeed.
But I'm a bit surprised to see that f_copy() works, as it should give the same warning as with f_default().
Concerning the case 2, AFAIK it's because of some reference magic: when you return $this->v as reference, it turns in refernce itself (refcount++) and the following call to f_default() returns this reference.
Though, it would be interesting to see what engine gurus say about it.

> In your case f_default() returns *copy*, not a reference   
No it doesn't.   
   
Try this as case 1 in the reproduce code as above: 
-------------------------------------------------- 
 case 1:   
   //forced reference works AND makes default behaviour 
   //work afterwards   
   echo incr($foo->f_ref())."\n";   
   echo incr($foo->f_default())."\n";   
   echo incr($foo->f_default())."\n";   
   break;   
   
Expected result:  
----------------  
1  
2  
2  
f_default() should return a copy 
  
Actual result: 
-------------- 
1 
2 
3 
incr() gets a reference to $foo->v and modifies it 
 
Since I filed this behaviour as another bug here 
http://bugs.php.net/bug.php?id=32840 
could it please be looked at too and be unbogusified?

What you are trying to do is feasable in  different ways. - just not the way you are trying.. (try the support channels for suggestions.) 
- whether what you are trying to do is a good idea (maintance, code readability, creating bugs....) is another question...

changing the engine behavior to do what you say is extremely complex. And the return on fixing this would not be worth the instability that it would involve (we know from previous efforts at looking at this).

hint: use
return &$this->var; 
$tmp = &$foo->f_default();
incr($tmp);