I ran the full test suite on "x86 Tiger custom" on my features/regrtest_timeout repository: no failure (test_threadsignals failure was not reproduced).

On "PPC Leopard custom", the test suite was interrupted (timeout) on test_io.test_basic_io:
----
...
[ 53/354] test_xdrlib
[ 54/354] test_descr
[ 55/354] test_io
Current thread 0xa09e1820:
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/codecs.py", line 302 in decode
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/_pyio.py", line 1378 in decode
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/_pyio.py", line 1682 in _read_chunk
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/_pyio.py", line 2002 in readline
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/test/test_io.py", line 1964 in multi_line_test
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/test/test_io.py", line 1945 in test_basic_io
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/case.py", line 387 in _executeTestPart
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/case.py", line 442 in run
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/case.py", line 494 in __call__
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/suite.py", line 105 in run
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/unittest/suite.py", line 67 in __call__
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/test/support.py", line 1078 in run
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/test/support.py", line 1166 in _run_suite
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/test/support.py", line 1192 in run_unittest
  File "/Users/buildbot/buildarea/custom.parc-leopard-1/build/Lib/test/test_io.py", line 2845 in test_main
  File "./Lib/test/regrtest.py", line 1029 in runtest_inner
  File "./Lib/test/regrtest.py", line 823 in runtest
  File "./Lib/test/regrtest.py", line 649 in main
  File "./Lib/test/regrtest.py", line 1604 in <module>
make: *** [buildbottest] Error 1
program finished with exit code 2
---
http://www.python.org/dev/buildbot/all/builders/PPC%20Leopard%20custom/builds/6

Cool! The traceback was printed correctly, the process exited correctly , and the build stopped also correctly. The full (interrupted) build took 27 min 41 sec, it takes usually 56 min 44 sec.

So a timeout of 5 minutes on each test (each function of each test file) is not enough on PPC Leopard buildslave. Anyway, I plan to use something closer to the actual buildbot timeout (1200 or 3600 seconds). And it would be better if the buildbot can choose its own regrtest.py timeout.