These aren't the drones you're looking for, you can go about your business. Move along.

There are a couple of companies that make WiFi roam quite well.
Amongst others...

RoamAD (http://roamad.com/) is a good example of a commercial company with rollouts that make WiFi roaming work.

Cohda Wireless (http://www.cohdawireless.com/) is a startup that also has a roaming solution, as well as some novel solutions to the non-line-of-sight problem that you mention. Actually, increasing the power doesn't fix the non-line-of-sight performance. The multipath increases with the power, so by boosting the power, you are also boosting the multipath components. The problem is solvable with some clever receiver algorithms which are still 100% wifi compatible.

A number of handset manufacturers are making handsets which switch between gsm/3g and a local wifi hotspot. Say you want to download a bunch of data (emails, work documents, etc). You head to the nearest hotspot and the handset switches to wifi via the internet (still maintaining the connection). You grab the data you want and then keep driving. This, as well as voip via wifi, will definitely take the cream out of the cellular profits.