There's a megaupload of the game over at the forum, which is about 290MB. A website called Superior Version has artwork and screens of it. I remember the official Dreamcast magazine previewing this (a scan of which is also at that link). I'm going to be giving this a shot, and I might even get a video of it up at some point if I get the time. With all the unreleased games that have some how snuck out of the woodwork and onto the internet over the years, I hope we'll get a leak of Dee Dee Planet one of these days..
EDIT: First Impressions! Well it's different, a sort of mix between a first person shooter with a huge rocket powered motorbike to fly about with, not a racing game at all. The bike is very twitchy, if you thought Daytona 2001 was hard to steer you've seen nothing. Good thing there's a analog sensitivity option then. You can make the bike jump and you have a whole bunch of weapons to flick through from a standard machine gun to rockets and these ice blade thingys.
I'm not really sure what the objective is yet, I must be in a hub world or something which I keep going around in circles in, killing the odd creatures walking about that spit explosive booger at you, and some rock monsters and rocker launcher machines. It has a story mode, time trial, a deathmatch mode and a gallery for unlocking artwork. It also has a rather interesting FMV opening. The cut scenes in game seem to fluff up a bit though.
EDIT 2: Here's the opening video of the game, uploaded to Youtube by Dreamcast.es.