I agree with Andy, you should really ensure that it's explained to your guests that their traffic is in the clear and it's their responsibility to protect their own data.
I'm not a lawyer, but just about every hotspot out there (hotels, Mcdonalds, Burger King, Panera Bread, Atlanta Bread, etc.) uses no encryption and they don't seem concerned about being held liable, and I'm sure their lawyers hashed that out a long time ago.
We considered using WPA-PSK, but the overhead would be massive for us. We have guests that can't even figure out how to connect to an open SSID. FYI, even though we didn't go a PSK route anyway, we didn't consider WPA2 because not all clients support it. Slowly but surely the ones that don't will fade away, but you'll end up with a guest that has an old client and can't do it.