While this doesn't sound so spectacular at first (nothing prevented you from doing so yourself up to now), it's actually great news for Prototype as it's part of a much larger project recently undertook by Mozilla.
You can read more about this project over at John's blog.
The benefits for Prototype, and for the other libraries which will be included shortly, are obviously huge. We'll be made aware of unavoidable regression bugs really early in the developing process and we'll probably avoid many which could have slipped unnoticed without this undertaking.
If we're lucky, this initiative will push other browser vendors to jump in the bandwagon. And we'll be there to help them make it possible should they decide to do so.