This is excellent... I think this looks like a great proposal and package to have.
The only issue I have is with the install directory and/or the require_once statements. Because the require_once statements all begin with 'Debug/ and not PHP/Debug the user is required to ini_set to an additional directory besides their PEAR dir and the changes seem a little troublesome.
My vote isn't conditional, as debugging is a special mode people work in which might not be usually in your standard include path - just something I noticed when installing and running your examples. You might consider changing all the require_once 'Debug to require_once 'PHP/Debug if the intended base install directory is PHP.
Great work, and a very useful addition... let's get this in the repository.