As of this morning the product I have been focused on for much the last year or so has at last been released: the Xbox 360 HD DVD Emulator. This is based on the Xbox HD DVD player, but designed for content authors to test and debug their titles without having to burn a disc. Content is created on a PC as usual, then you run it on your regular Xbox 360 under the emulator by sharing it over the network, putting it on a USB drive/stick, or burning an actual disc. A Viewer application runs on the PC to collect the information from the title as it runs, allowing the author to determine what happened and why (or maybe why not).
An emulator has the exact same feature set as a real player, but with extra hooks to allow authors to "see into" the ECMAScript and markup as it runs. This is in contrast to a simulator (such as our PC-based Jumpstart Kit) which is not fully featured (Jumpstart can't play real HD DVD encoded audio/video and is based on the old, original Toshiba A1 HDi codebase). Note that the emulator cannot run AACS-protected titles as it is designed for pre-production use. It does allow titles to run in full-trust mode as if they were AACS titles, so you can access pstorage and the like without making a real AACS disc. (The emulator's pstorage is separate from the player's pstorage of course).
Here is a screen shot of the Log Viewer in action: it is showing the startup sequence of Peter's Liar's Dice sample. The grey section shows the script execution path: which file, which function, and the variables as they change.
As an aside, the PC components were my first real code written in C#, and I have to say I'm a huge fan of my old team's work. Intellisense rocks, and WinForms make it easy even for a UI-clunker like me to make a decent looking application. Plus of course the debugger is fabulous...