The full version of MPiD isn't out yet, but you can still preview it in one of two forms: The Beta, and the Demo.
The Demo doesn't follow the story of PiD; in fact, it doesn't have a story at all. There are three levels, loosely based on the original game's architecture, and littered with hundreds of monsters for you to massacre. You'll notice the interface is not particularly PiD-like, as well; the save points are Marathon-style pattern buffers, for example.
The Beta represents the current progress towards the completion of the full game. It's broken in quite a few places and there are obviously incomplete levels, but you can still follow the storyline to the degree that it's implemented. Many of the lingering Marathon-isms have also been removed from the gameplay, or altered as much as the Aleph One engine currently allows (which, sadly, is not quite as much as we'd all like, but such is life).
If you don't care about the plot and just want to shoot things, the Demo is probably your best bet. If you care more about the PiD story than whether doors actually open and close when they ought to, then go right ahead and fire up the Beta (and perhaps tell us about the bugs you find). If you want both robust gameplay and faithfulness to the story and style of PiD.... well, you'll just have to be very patient. ;)

Beta Released August 11th, 2007

Download from Rapidshare (43.5 MB)

Demo Released April 9th, 2005

Download from archives (18.2 MB)

Download via FTP

Note: According to some users, playing the Demo on Windows requires deleting the MML Script file from the Scripts folder.
While no reports have yet come out related to damage caused by MPiD, please continue to keep in mind that the MPiD Team is in no way responsible for your usage of this demo and any resultant damage to your system or data.