Depends on the store, if they're busy they may require it.
There are Apple stores that aren't busy?
I'm looking to get my iPhone replaced because of poor battery life (3 hours+ usage) do I have to book a Genius Bar appointment for this?
no, you don't need an appointment. I just did mine yesterday. swapped a black 16GB for another Black. though it did not go smoothly because there was an issue with verizon exchange. I had to do a return, lose number, and add another line.
Wrong they do need one or they will either get denied or have to wait for hours.
Both of the ones around me are normally busy but if you go in at the right time, they're sometimes not.
Genius appointments are cake to make. Question is, why wouldn't you make one?