Concerns around the effects that too many users may have on Wi-Fi networks seems to be the hot topic these days: This writer suggests a somewhat unclear idea revolving around a "congestion charge" and technology that will turn off clients that aren't being used.
I just don't think congestion is going to be that huge of a problem. It will be in certain circumstances like at trade shows and possibly on a busy street where every shop offers Wi-Fi. But otherwise I just don't expect it to be debilitating.