Sorry to repost comments from another thread but they seem relevant here. At my home sitting next to my router my WiFi speeds are 10-15 times faster than my 3G speeds where I seldom show more than 2 bars for my cell phone network connection.
On the other hand, I was on a trip a couple of weeks ago staying on the top floor of a small hotel that offered "free wifi." My 3G speed (both on my phone and my laptop) was about twice as fast as my wifi speed. My assumption is that the weak wifi signal on the top floor (without a repeater) and unknown speed of the hotel's connection to the internet accounted for the difference.
In short, like many questions, the answer to this one is "it depends." Interestingly, though, even when the speed difference is huge, as it is in my home, the user experience is not that different. (Of course, I'm not watching movies on my phone, either.)
Finally, as someone else noted, the use of wifi for internet tasks does make it possible to duplicate the iPhone's (GSM) ability to web surf and sit on hold at the same time. In that vein I find it amusing that AT&T has finally found a realistic situation in which being on the phone and accessing the web simultaneously is worth mentioning in their advertising. I'm not sure, however, that reminding people of the delights of being on hold for an extended period is the best advertising for a phone, especially as one uses precious phone minutes in the process.