The laptop dock is an actual laptop/netbook "shell" where all the processing and data files IS the atrix itself.
It looks like the cradle you found is just a "stand" for the phone that allows you to connect via usb to your phone. I doubt there is any difference between that and using the usb cable that came with your phone.
This the laptop dock:www.amazon.com/AT-Laptop-Dock-Motorola-ATRIX/dp/B004M17D62
As for "bypassing" the rebate to get it immediately for $50 cost, I believe you can try to negotiate a "price match" at a brick & mortar att store.
I assume you see the rebate offer from an online source.
Also... I've heard that att stores either sold out and don't carry/won't order additional laptop docks :-(
Sent from TapaAtrix