The fine folks at ComputerMinds recently posted a modified SimpleTest run-tests.sh script for running SimpleTests from the command line. Their script added an --xml option to allow the script to run tests and output results in JUnit's XML format so that Hudson can automatically run all SimpleTests in your project.
SimpleTest is also included inside of Four Kitchens' Pressflow distribution. The latest version of SimpleTest is 6.x-2.9, which you will also get if you upgrade your version of Pressflow past bzr revision 49. Attached to this post is an upgraded version of the run-tests.sh script suitable for use with SimpleTest 6.x-2.9, regardless of if you are using Pressflow or not.
The relevant d.o issue is at http://drupal.org/node/602332, but the active discussion around this issue is already happening in Pressflow's Launchpad project where some of the team working on the Economist are going to compare notes shortly.