has anyone had any luck finding a good vent or window mount cradle that fits a DHD with otterbox around it?
No, but I'm using a Dash Genie from mobilefun.co.uk right now, and it seems to hold the DHD in the Otterbox Commuter well enough. I was concerned the phone would be too heavy for it, but it actually holds better in the Otterbox case than it does with the phone naked - perhaps the case makes the back flatter. It's not rock solid but it's pretty stable, at least I haven't knocked it off yet!
I've also dropped the phone a few times since I put this case on, so I'm very glad I bought it. I don't feel it adds much to how big the phone feels in my hand. It's slightly thicker, but the camera feels much more protected. The power button is easier to find, but a bit spongy, so not always easier to actually press. Overall I'm very pleased with this case, but if I had to pick any faults with it they would be the buttons, which stick out slightly and look a bit naff (I may pluck up the courage to trim them as posted earlier), and the strip of silicon at the bottom of the screen, above the USB socket. It's the only bit of the inner case that isn't firmly clamped down by the hard shell, and very occasionally lifts up. Oh, and the silicon tends to pick up fluff from my jeans pocket, but it doesn't adhere and wipes off in seconds. The screen protector is good, but I've ordered a Brando antiglare one after seeing Andy's review.
But as I said, I've dropped the phone a few times already, and have been very glad of the extra protection. (I use a wheelchair, so my hands are always busy when I'm moving around. Phone on lap = phone likely to be dropped.

) I think it's a really good compromise of protection and appearance, so very pleased with the purchase.
To the guy asking about the CaseMate Barely There - I haven't seen the DHD one, but had one for my iPhone, as I could leave it on while putting the phone in my TomTom cradle. It protected the back of the phone against scratches, but didn't offer anything like the protection the Otterbox does. However it hardly added any extra size or weight to the phone, sot they're very different cases aimed at answering different needs.