With most WiFi encryption (basically except WPA2 Enterprise, which won't be in use in cafés) you can still receive the traffic as long as you see the initial handshake for the device connecting to the network.
Sniffing software like Wireshark or Aircrack-NG can handle the decryption transparently.
Once you can see the traffic Firesheep or something like it will work in the same way as on an unencrypted network.
With most WiFi encryption (basically except WPA2 Enterprise, which won't be in use in cafés) you can still receive the traffic as long as you see the initial handshake for the device connecting to the network. Sniffing software like Wireshark or Aircrack-NG can handle the decryption transparently.
Once you can see the traffic Firesheep or something like it will work in the same way as on an unencrypted network.