Monitoring my Airport Extreme base station (2.4 GHz WPA2) my Apple Watch Sport will be listed in Wireless Clients as 802.11b/g/n. I can identify it by the WiFi Address as listed in the Watch iOS app under General -> About. Simply turning Bluetooth off via the iPhone's control panel will cause the Watch to jump onto WiFi.
When connected to WiFi, all functions work, including receiving a voice call placed via my home landline to my iPhone and answering it on my Watch (surprisingly low latency and excellent call quality). Most importantly, pinging the iPhone works.
I have plaster walls in this 100 year old house, so my WiFi signal on 2.4 GHz doesn't go very far. I may install some repeaters to help with that. 2.4 GHz is the same frequency used by microwave ovens, so my or a neighbor nuking a cup of coffee can take down or slow down WiFi and Bluetooth connectivity.
I wouldn't expect the Watch to have a powerful WiFi transmitter, which would limit its maximum range from the base station, but who knows.
Now, what would be cool would be WiFi connection to the cloud / phone on different networks, so if I left my iPhone at home, I could still receive services on my Watch via the gym's WiFi coverage. I think Android Wear is soon to offer that... would be nice to see Apple do similar.