I'd like to offer you a quote from Mark Shuttleworth on this:

In his most recent article "Discussing Free Software Synchronicity" http://www.markshuttleworth.com/archives/150

A comprehensive test suite, on the other hand, lets you be more open to big landings on trunk, because you know that the tests protect the functionality that people had *before* the landing. A test suite is like a force-field, protecting the integrity of code that was known to behave in a particular way yesterday, in the face of constant change. Most of the projects I’m funding now have adopted a tests-before-landings approach, where landings are trunk are handled by a robot who refuses to commit the landing unless all tests passed. You can’t argue with the robot! The beauty of this is that your trunk is “always releasable”. That’s not *entirely* true, you always want to do a little more QA before you push bits out the door, but you have the wonderful assurance that the test suite is always passing. Always.

Building code is easy, building a culture is hard.

-Hank