It depends. Obviously unencrypted wifi is trivial to sniff, and WEP encrypted wifi is only modestly harder. However, wifi can be configured to be highly secure, with for example WPA2/AES, etc.
Both GSM and CDMA are encrypted, but I believe both have been broken and aren't considered as secure as something like WPA2/AES.
Keep in mind though, that it's not just the trip over the wireless network that's at issue, it's also anywhere on the Internet between you and the service you're using (Foursquare in this case).
Both GSM and CDMA are encrypted, but I believe both have been broken and aren't considered as secure as something like WPA2/AES.
Keep in mind though, that it's not just the trip over the wireless network that's at issue, it's also anywhere on the Internet between you and the service you're using (Foursquare in this case).