I've seen the topic of bike borrowing pop up on HU and ADVrider several times. I think it's a great idea but very challenging to accomplish on the forums. I'm looking into building a site dedicated to bike borrowing - would you be interested?
I put together a simple landing page to judge interest. If you think it might be something you're interested in, please submit your email address. If not, that's fine too.
RiderMix - Borrow A Bike Anywhere In The World
Obviously there are more than a few logistics issues to be solved. Questions? Comments? Send me a note (I don't want to hijack this thread).