Airplay is the next generation of the Airport Express system that Apple has had for years. It is wifi (or cabled) which means it has much more available bandwidth than bluetooth. The new AppleTV will also support video rather than just audio. Sure, a set of bluetooth speakers would likely work, but you'd have limited choices, and be limited to a range of about 30'. Airplay devices can be stand alone, to connect to ANY stereo, or built in by an OEM.
The real advantage of Airplay is that you can have multiple devices and control them from any Mac computer or iOS device on the network. You can have multiple devices and stream the content to one device, some, or all, and have individual volume control. With bluetooth you would be paired with one device at a time. Also, in my experience, bluetooth connectivity is not terribly reliable. That wouldn't be so great for something like a phone where you are walking in and out of range of the speakers all the time - if you were streaming with bluetooth, you'd have to stay within range of the speakers to keep your music playing.
I've had several Airport Expresses connected around the house for years - it's one of Apple's great 'secret' products that is about to get a whole lot better.
