Joseph has links to some pictures of the Knights Armament PDW.  Looks like Knights went with a more traditional approach, and basically has produced a variant of the standard assault rifle, only with a shorter barrel and firing an intermediate cartridge. One advantage to going this way is that the rifle can have iron sights.
I’m more of a fan of the compact PDW platform. I’d rather see a bullpup style config, with the trigger forward. I think a certain compactness should be key to the PDW concept.
I personally like the P90, you just can’t get the good ammo here; plus, the Knight’s an extra 100 meters of effective range against body armor. If they ever release an SBR version for us lowly civvies, I’d definitely buy one (providing that I had the cash).
I had a couple of the original Bushmaster Armpistols back in the early ’70s.
http://www.biggerhammer.net/armpistol/
The prototypes for the Air force were in 221 Fireball rather than .223 and in that caliber would still make a nice PDW for those who want a wimpy weapon.
One in 10mm would do everything I could want.
I rather liked Ahab’s plan to put a bull-pup stock on a Ruger 10/22. Sure, it may not pack much punch, but it should be relatively handy and accurate.
People who favour the bullpup try using one -for real. If you’ve got to dash out of a car you want compact – and bullpup gives you that extra barrel length. But the ergonomics suck – subcarbines are generally last ditch territory – you are going to be using every mag you’ve got. Try keeping ready when you’ve got to fumble in your armpit to change. Hey, I lose accuracy and a bit of punch with a conventional mag port. I’ll _live_ with that to keep firing. Knights/TSWG have selected a more powerful round for CQB – still got enough reach to tackle your average AK wiggler. I’ll go with that – sadly I think top brass and the accountants won’t. Wonder what it takes for one to fall of the back of a humvee in Florida?