Here's more information from their press release:

Blue Static today is announcing the release date of MacGDBp, a native Cocoa application that allows Web developers to debug their PHP applications. The tool makes use of the Xdebug (http://www.xdebug.org) PHP extension that provides remote debugging functionality. Blue Static will be releasing the software under the GNU GPL version 2 on the morning of Tuesday, 17 June 2008.

The interface for the application is modeled after the XCode product from Apple to help integrate it more fully into the OS look and feel. It will require Mac OS X 10.5 Leopard to run and screenshots of the tool can be found here and here.