I made this DIY iPad Car Mount video on the day I got my iPad 3G and I forgot to post it here. After 3days, I found there are over 8000 views from youtube. I thought you might be interested in it as well.
This video shows you how I managed to use a string to hold the iPad on my car dashboard. This solution should be able to be applied to most cars unless your car has special designed dashboard. It holds the iPad very firm and allows you to remove the iPad easily.