The patch solves one issue. It resets the Py_DefaultFileSystemEncoding
to NULL when no default value was given. However the finalization fails
after the 3rd round at 

        if (op == &refchain ||
            op->_ob_prev->_ob_next != op || op->_ob_next->_ob_prev != op)
                Py_FatalError("UNREF invalid object");

$ gdb ./reinit_test
GNU gdb 6.6-debian
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This GDB was configured as "i486-linux-gnu"...
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
(gdb) run
Starting program: /home/heimes/dev/python/py3k/reinit_test
[Thread debugging using libthread_db enabled]
[New Thread -1210464064 (LWP 8060)]
round 1
[23832 refs]
round 2
[24558 refs]
round 3
Fatal Python error: UNREF invalid object

Program received signal SIGABRT, Aborted.
[Switching to Thread -1210464064 (LWP 8060)]
0xffffe410 in __kernel_vsyscall ()
(gdb) bt
#0  0xffffe410 in __kernel_vsyscall ()
#1  0xb7dc7875 in raise () from /lib/tls/i686/cmov/libc.so.6
#2  0xb7dc9201 in abort () from /lib/tls/i686/cmov/libc.so.6
#3  0x0805d62d in Py_FatalError (msg=0x81790d8 "UNREF invalid object")
at Python/pythonrun.c:1761
#4  0x080aac57 in _Py_ForgetReference (op=0x821d728) at
Objects/object.c:1536
#5  0x080aaca6 in _Py_Dealloc (op=0x821d728) at Objects/object.c:1555
#6  0x080c0f52 in tupledealloc (op=0x82ebc3c) at Objects/tupleobject.c:169
#7  0x080aacb1 in _Py_Dealloc (op=0x82ebc3c) at Objects/object.c:1556
#8  0x080c0f52 in tupledealloc (op=0x83f9154) at Objects/tupleobject.c:169
#9  0x080aacb1 in _Py_Dealloc (op=0x83f9154) at Objects/object.c:1556
#10 0x08083605 in code_dealloc (co=0x83b1bc8) at Objects/codeobject.c:276
#11 0x080aacb1 in _Py_Dealloc (op=0x83b1bc8) at Objects/object.c:1556
#12 0x081691bf in func_dealloc (op=0x83f7924) at Objects/funcobject.c:555
#13 0x080aacb1 in _Py_Dealloc (op=0x83f7924) at Objects/object.c:1556
#14 0x080a272e in PyDict_Clear (op=0x83f6df4) at Objects/dictobject.c:798
#15 0x080a524f in dict_tp_clear (op=0x83f6df4) at Objects/dictobject.c:1793
#16 0x08068c13 in delete_garbage (collectable=0xbf9fa024, old=0x81978e8)
at Modules/gcmodule.c:688
#17 0x0806909f in collect (generation=2) at Modules/gcmodule.c:824
#18 0x080699ea in PyGC_Collect () at Modules/gcmodule.c:1238
#19 0x0805a022 in Py_Finalize () at Python/pythonrun.c:416
#20 0x08059791 in main () at reinit_test.c:9
(gdb)