I've been looking for something like this since I first obtained my Kindle. I previously had a library card from another state and they had a fair selection of ebooks that I could download. Currently, my local library has zero ebooks.
If I happened to live in New York, I could get a library card and have access to over 13000 ebooks. Cleveland has a similar number. Other libraries have less, often none at all. It's a shame that I get nothing because I live in a particular area.
A universal ebook lending library would be very interesting to me. Overdrive does this for existing libraries:
http://www.overdrive.com/products/library.asp
Patrons would have to pay to support the system somehow. Maybe members could buy an ebook through the library as a proxy, automatically take it out for loan, then when the lending period expires, the library would own it and the donator would receive credit for X number of new check-outs. Or maybe just a flat rate for X number of check-outs similar to the Netflix model.
Overall I think it's a good idea.