Take, for example, the iPhone. Without Wi-Fi connectivity, the more popular services such as YouTube and iTunes streaming wouldn't have been possible. Given that with 3G this might be eliminated, but with speeds averaging only 500K-1M (if you're lucky), you'll most likely see choppy videos and incomplete music sampling.
T-Mobile is finally doing what I've thought all carriers should have done a long time ago, which is offering the option to connect your mobile number and services to your home's Wi-Fi connection to eliminate the need for both a home LAN line and the need to use your ever-so-valuable voice and data services from your mobile carrier while you're at home. For T-Mobile, this is a win-win situation. It's reducing the load on its network while cementing its service into consumer's minds as their only means of voice communication wherever they may be, whether at home or abroad.
The Wi-Fi alliance, which keeps tabs on the industry, recently estimated that between 250 and 350 million Wi-Fi/mobile converged devices will be in the market by 2011. This solidifies the future trend of using voice-services over Wi-Fi when available, or at least until other mobile broadband technologies catch up. With such a large number being produced, there will undoubtedly be a need for Wi-Fi in mobile for a long time to come.
Wi-Fi obviously has its limitation when considering vast access-deployment for mobile devices, but until things improve, it's easy to forget that Wi-Fi has been powering mobile device data communications and services for several years now without a hitch.