I am also hitting this issue.

I have been able to reproduce it in python 2.5 and 2.7, on debian 6 (64 bits)
Exactly like the reporter, I am making extensive use of threads. I am also using SQLalchemy.

All my threads but one are waiting (probably for the GIL) :
(gdb) info threads
  15 Thread 0x7fffe3ff7700 (LWP 419)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  14 Thread 0x7fffe47f8700 (LWP 418)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  13 Thread 0x7fffe4ff9700 (LWP 417)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  12 Thread 0x7fffe57fa700 (LWP 416)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  11 Thread 0x7fffe5ffb700 (LWP 415)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
* 10 Thread 0x7fffe67fc700 (LWP 414)  0x000000000046479f in _Py_ForgetReference (op=0x4dc7bc0)
    at Objects/object.c:2222
  9 Thread 0x7fffe6ffd700 (LWP 413)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  8 Thread 0x7fffe77fe700 (LWP 412)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  7 Thread 0x7fffe7fff700 (LWP 411)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  6 Thread 0x7fffec81c700 (LWP 410)  0x00007ffff72a11a3 in select () from /lib/libc.so.6
  5 Thread 0x7fffed01d700 (LWP 409)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  4 Thread 0x7fffed81e700 (LWP 408)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  3 Thread 0x7fffee01f700 (LWP 407)  0x00007ffff7bd03c0 in sem_wait () from /lib/libpthread.so.0
  2 Thread 0x7fffee820700 (LWP 406)  0x00007ffff729c9d3 in poll () from /lib/libc.so.6
  1 Thread 0x7ffff7fec700 (LWP 393)  0x00007ffff72a11a3 in select () from /lib/libc.so.6

And the segfault happens in _Py_ForgetReference :

(gdb) bt
#0  0x000000000046479f in _Py_ForgetReference (op=0x4dc7bc0) at Objects/object.c:2222
#1  0x0000000000464810 in _Py_Dealloc (op=0x4dc7bc0) at Objects/object.c:2242
#2  0x0000000000559a68 in frame_dealloc (f=0x4997ab0) at Objects/frameobject.c:458
#3  0x000000000046481d in _Py_Dealloc (op=0x4997ab0) at Objects/object.c:2243
...

probably because something else zeroed out the op object :
(gdb) select-frame 0
(gdb) print *op
$6 = {_ob_next = 0x0, _ob_prev = 0x0, ob_refcnt = 0, ob_type = 0x2364020}

Is there something else I can do to help resolve this bug ? I can help debugging this, but I am not sure how to proceed. I have a --with-pydebug interpreter stopped at the segfault in gdb.