The old tap and drag to mass select photos has been reassigned to rearranging photos. So it isn't obvious how to select large groups of photos anymore. Many people have commented on the Apple Discussion Forums having trouble with this.

The new method is simple but not so intuitive. Just use two fingers, close together to tap on a photo, hold for a second and then drag across all the photos you want to select.

It works just like the old method, just needs two fingers instead of one.

[crarko adds: I tried this with the iPhone but I think you'd need far smaller fingers than mine to do this.]