http://finance.yahoo.com/news/NFC-Mobile-Phones-Replace-iw-182820489.html?x=0&.v=1 http://www.assaabloy.com/com/Products/ASSA-ABLOY-Mobile-Keys/
I doubt you could just download the key to another phone, but I would guess that they can authorize as many NFC devices to each room as needed, just like they do with the keycards--usually you get two, but you can ask for more. I'm sure there's some kind of reasonable upper limit though. Letting the user distribute the key doesn't sound very secure--I'm sure there would be a virus out to hijack keys much too quickly.
Great stuff. Soon we only need our phones for everything. I need to read more about how the carriers make money and what the rate %'s are for this type of service or is this just a way for carriers to increase there sms/mms media? Interesting...