Last year when Apple announced iOS 5 it also unveiled a couple of new services. iTunes Match was one of them. In iOS 5 iTunes Match downloads songs to your device, it could not stream them. In iOS 6, it has been discovered that iTunes Match streams songs instead of downloading them.
iOS 5 did not bring the ability of music streaming via iTunes Match. This was possible on Mac computers but not on iPhone, iPad or iPod touch. It has been confirmed now that iOS 6 enables iTunes Match streaming on iOS devices as well, as opposed to downloading of tracks on iOS 5.
Right now only the first beta of iOS 6 has been seeded. It is not unusual for Apple to give some features in developer versions but pull them out of the final public release. If iTunes Match streaming is set to be a permanent feature in iOS 6, then it will be available in the GM build which comes out days before the final public release in Fall 2012.