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I spent some time bisecting the SVN history in the py3k branch, and believe that r84914 is the commit that introduced this issue.
Details:
 
 Trying on 4-core i386 RHEL 5 box
 $ svn up -r REV
 $ make clean ; make
 (configured --with-pydebug)
Reproducer:
 ./python -c "import multiprocessing.managers ; mpp = multiprocessing.Pool(4); sm = multiprocessing.managers.SyncManager(); sm.start()"
 (running it 3 times; each time, at each build I see it either successfully completing 3 times, or having the Py_FatalError 3 times)
Bisecting:
 r86720: CRASHES with r86729 --with-pydebug (from when I was investigating this before)
 r73573: Runs OK with r73573 --with-pydebug (r73573 was origin for 3.1 tag)
 => somewhere in 73572..86729
 r76195: Runs OK with r76195 --with-pydebug (r76195 added the new GIL)
 => somewhere in 76195..86729
 r80724: Runs OK with r80724 --with-pydebug (changed threading code to "Make (most of) Python's tests pass under Thread Sanitizer.")
 => somewhere in 80724..86729
 r83722: Runs OK with r80724 --with-pydebug (touched multiprocessing)
 => somewhere in 83722..86729
 r85222: CRASHES with r85222 --with-pydebug (rough midpoint)
 => somewhere in 83722..85222
 r84472: Runs OK with r84472 --with-pydebug (arbitrary midpoint)
 => somewhere in 84472..85222
 r84847: Runs OK with r84847 --with-pydebug (arbitrary midpoint)
 => somewhere in 84847..85222
 r85033: CRASHES with r85033 --with-pydebug (arbitrary midpoint)
 => somewhere in 84847..85033
 r84938: CRASHES with r84938 --with-pydebug (arbitrary midpoint)
 => somewhere in 84847..84938
 r84892: Runs OK with r84892 --with-pydebug (arbitrary midpoint)
 => somewhere in 84892..84938
 r84915: CRASHES with r84915 --with-pydebug (arbitrary midpoint)
 => somewhere in 84892..84915
 r84903: Runs OK with r84903 --with-pydebug (arbitrary midpoint)
 => somewhere in 84903..84915
 r84914: CRASHES with r84914 --with-pydebug (affects threads)
 => somewhere in 84903..84914
 r84913: Runs OK with r84913 --with-pydebug (previous commit before 84914)
 => r84914 is the commit that triggered this issue