If I need to use my iPad for navigation while driving, I currently just put my sun visor down and slip the original iPad case over as posted previously. That works great and no string required. Also great for passengers since back seat riders are able to watch videos with it positioned that high.
Check out this custom option I came across recently at labshield.com
It's an iPad kiosk made from stainless steel and features:
VESA bolt pattern for countertop or wall mount
iPad speakers, mic, wifi, and 3G remain functional
Secure iPad buttons to lock-in your app or website
Currently...
There's a nice app called Mailing List that can help you collect emails at trade shows. It plays a custom slide show and when the user taps the screen, it opens up fields for the user to put in their name and email. It works best if a display box could be custom made to cover up the home key...