That's really interesting. I was chatting with a neighbor who happens to be an executive at one of Audi's US affiliates about the growing number of people in the US who don't want to drive and the growing number of young people here in the US don't want to own cars.
We are both finance guys, and the issue that immediately came to mind is how to determine an amount of rent when cannot predict the residual. There are no usage models or other predictive data to allow reasonable estimates as to usage, useful life, wear and tear, etc., so this would be a huge problem for a company trying to do this on a large scale.
The system BMW is proposing does away with this problem (or more specifically disperses the problem and shifts the financial risk to the person who owns the rental car).
One huge issue with doing this in the US is that the rental fees, no matter how small, are taxable. I predict we will soon see an information reporting requirement for any such payments so that the IRS can track them.