Here's an interesting comparison of how much spectrum is used today for 3G vs. Wi-Fi. In Europe, UMTS/HSPAS is assigned a 60 MHz uplink + 60 MHz downlink = 120 MHz. Out of that only 20+20 = 40 MHz are in active use today even in dense urban areas except for a few exemptions. For details, see here and here. In the future, the digital dividend band in the 800 MHz band (2×35 MHz), and the IMT extension band in 2.6 GHz (2×70 MHz) will be added.
Wi-Fi 802.11b and g use the 2.4 GHz band spanning 80 MHz. Especially in dense urban areas, I'd say that's already used quite intensively these days. In addition, some 802.11n kit can also use the 5 GHz band, where almost 400 MHz is allocated for Wi-Fi and other short range wireless systems. It's probably still not used a lot but with more and more 5 GHz band enabled equipment appearing that will certainly change.
So, what's the message here? There are several. 3G only uses a fraction of the spectrum that is already allocated today without suffering massive bandwidth bottlenecks. The same applies to Wi-Fi although the original 2.4 GHz band is already quite busy. But if your gut feeling told you that cellular networks probably had much more spectrum available than Wi-Fi, then that was not quite accurate.