I love the idea. A smartphone has so many capabilities these days. I turn off WiFi and Cellular service on mine when we leave port but I still use my phone for a lot of things. It's a convenient camera, clock, gaming, kindle reader, cruiseline app with events and such, ship map, vacation notes (I store notes in OneNote leading up to the vacation on things I want to do or have planned) and now it could even be my room key.
I'd love to bypass the lines and get to my room. If even 10-15 percent of people took advantage, it would drop the lines for everyone else too. I'd be surprised if they are using bluetooth technology as that usually requires some sort of pairing and you don't want to have to do that. They are probably using NFC, which most smartphones now have. It's how the payment apps work. NFC is Near Field Communications. It allows devices to "talk" to each other when in very close proximity.
If you have a newer car, you might have one of those keys that you can just leave in your pocket or purse. The car has sensors and when it notices it's key, it will light up. When you touch the door handle, it will unlock. If the key is in the car, you push the button to start the car. My car doesn't even have a keyhole on the exterior.
Point being, this technology is maturing very rapidly with many uses all around us. The point was made earlier that it has been in several hotels already. I'm sure cards will still be issued on request and they will likely need to have a backup plan for guests who lose their phones or have the battery die on them while on shore. It's possible for there to be a malfunction of some sort as well. I've had that happen with the card room keys too though, so I don't think the frequency would be much higher. All they are really doing is transferring the programming that is in those cards to the smartphone.
The can of worms that does get opened is the various ways that smartphones are configured and the wide range of knowledge people have about their smartphones. For example, when you set your phone to airplane mode when on the ship, it may disable the NFC function as well. Many smartphones disable bluetooth in airplane mode as well. Depending on what technology is being used (and I'm assuming NFC), a passenger could inadvertently disable the communication function and now Guest Services is becoming a help desk for your smartphone, or just issuing you a key anyway.