It's called "Zune", obviously in honor of Microsoft's upcoming iPod ripo... I mean "killer", and it's basically a black version of the WinXP Media Center theme with an orange start button (don't ask). The version being distributed by Microsoft though gets the directory structure wrong and the theme doesn't appear completely correct (the task panes in explorer windows aren't styled), fortunately someone managed to fix it, and that's the version I have (found in this thread). The best thing is that this is signed by Microsoft, so anyone can install it with or without a patched over uxtheme DLL. Here's a screenshot:

Edit: Some other points I forgot to mention:
-This theme is an improved version of the unfinished "Royale Noir" theme that was released a little while ago.
-Even though it's is based off of the WinXP Media Center theme, you don't need Media Center to install it, it installs just fine on XP Home (which I have) and presumably, XP Pro.
-The zip contains two executables, one installs the theme and the other installs the wallpapers (both wallpapers are the same image of an outdoor concert)