Hi,
I ma unable to reproduce this with master:

gkotton@openstack:~/nova$ tox
py26 create: /home/gkotton/nova/.tox/py26
ERROR: InterpreterNotFound: python2.6
py27 develop-inst-nodeps: /home/gkotton/nova
py27 runtests: commands[0] | python setup.py test --slowest --testr-args=
running test
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-160} \
${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests --list 
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-160} \
${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests  --load-list /tmp/tmphFdy35
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-160} \
${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests  --load-list /tmp/tmpNHjZBv
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-160} \
${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests  --load-list /tmp/tmpNJcdWX
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-160} \
${PYTHON:-python} -m subunit.run discover -t ./ ./nova/tests  --load-list /tmp/tmpVNZqv6
Ran 10051 (+8922) tests in 711.033s (+675.827s)
PASSED (id=1164, skips=11)
Slowest Tests
Test id                                                                                                   Runtime (s)
--------------------------------------------------------------------------------------------------------  -----------
nova.tests.integrated.v3.test_hide_server_addresses.ServersSampleHideAddressesJsonTest.test_servers_list  43.311
nova.tests.integrated.test_multiprocess_api.MultiprocessWSGITest.test_killed_worker_recover               42.130
nova.tests.integrated.v3.test_deferred_delete.DeferredDeleteSampleXmlTests.test_force_delete              27.586
nova.tests.integrated.v3.test_flavors.FlavorsSampleJsonTest.test_flavors_get                              26.525
nova.tests.virt.libvirt.test_libvirt.LibvirtConnTestCase.test_get_guest_cpu_config_host_model_old         26.148
nova.tests.integrated.v3.test_quota_class_sets.QuotaClassesSampleJsonTests.test_update_quota_classes      22.885
nova.tests.integrated.test_multiprocess_api.MultiprocessWSGITest.test_terminate_sigterm                   20.466
nova.tests.integrated.test_api_samples.ShelveXmlTest.test_shelve_offload                                  19.615
nova.tests.test_availability_zones.AvailabilityZoneTestCases.test_update_host_availability_zone           19.152
nova.tests.integrated.v3.test_flavor_rxtx.FlavorRxtxXmlTest.test_flavors_rxtx_create                      18.455
py33 create: /home/gkotton/nova/.tox/py33
ERROR: InterpreterNotFound: python3.3
pep8 develop-inst-nodeps: /home/gkotton/nova
pep8 runtests: commands[0] | flake8
pep8 runtests: commands[1] | /home/gkotton/nova/tools/config/check_uptodate.sh
__________________________________________________________________________________________________________________________________ summary ___________________________________________________________________________________________________________________________________
ERROR:   py26: InterpreterNotFound: python2.6
  py27: commands succeeded
ERROR:   py33: InterpreterNotFound: python3.3
  pep8: commands succeeded

Did I miss something?
Thanks
Gary