test-run --all fails to run after upgrading to drush 5.1

I recently upgraded to the 5.1 release of Drush via PEAR.
After the update test-run --all fails to actually run any tests. Specifying a test class works fine.
Looking at the processes being run it appears that the root and uri arguments aren't being propagated to the backend processes.
If I run the following command on the shell:
drush --root=/data/drupalroot/ --uri="mysite.example.com" -v test-run --all
It spawns processes of the following form:
/usr/bin/php /usr/share/pear/drush/drush.php --php=/usr/bin/php --backend=2 --root= --uri= test-run AggregatorConfigurationTestCase --no-all
The --uri and --root args are blank. If I copy the command and run it with missing values added the commands run successfully.
Aside from the problem with the arguments not being passed I would have expected the errors from the backend calls to be captured and reported back.