No one has concrete information. Not even 3D Realms. Seriously, they have repeatedly scrapped most or all of the plot, models, and art work. Trailers and art have been put out, only to be disclaimed months or years later when that plot/environment/character is written out. It might come out this year, or in 2009, or never. They're still using the Unreal Engine, for goodness' sake. I'd say the chances of it being scrapped and restarted are still high.
A game has a window of maybe three or four years before graphics passes it by. Anything in development beyond that point is going to look dated when it actually arrives. If you don't get it out by then, you either release it looking like it's old (okay if it's a really original and fun game or an indy) or just start over. If you start over, all your art and models have to be redone. If 3D Realms had gotten their story boards and plot finalized, then licensed the Unreal 3 (or equivalent) engine when it first came out and then didn't screw with the engine, but spent all their time making the game, art, models, etc. then it might be possible to get it out this graphics cycle and have it look okay, but from what I understand they are still re-writing major parts of the Unreal engine. WTF? I would think it would be clear to 3D Realms by now that they are no longer capable of competing with the big boys in such work; they simply fall further behind. This is a company that still sells games on floppy disks and they're re-writing game engines?
Assuming it doesn't end up being released for cell phones or the GameBoy where the Unreal engine might be cutting edge, I'll probably pick up a copy when it hits $20 if it doesn't totally suck. Maybe for $10 even if it does really suck. Then again, sometimes a clean break is best.