How do you spell wifi / Wi-Fi / WiFi?

Since you have an iPhone app and it would seem that Apple/iPhone use the term "Wi-Fi" throughout then I would use the same for consistency. iPhone users expect to see "Wi-Fi".

However, there could be a regional difference...

Note: iOS devices sold in China may use the term Wireless LAN (WLAN) instead of Wi-Fi.

Reference: http://support.apple.com/kb/ts1398

(EDIT: For comparison, my Nokia E65 (sold in the UK) uses the terms "WLAN" or "Wireless LAN" exclusively, no mention of "Wi-Fi" or its derivatives.)

In the past it was "Wi-Fi", but the current trend is toward "wifi".

It's a little like e-Mail => eMail => email.

December 2015 update: A Google search now results in 41% more occurences of "wi-fi" than "wifi". Apparently the trend has reversed.