pg has stated a preference for avoiding industries wherein one has to get permission from huge companies to even exist...but then again, Loopt is perhaps his favorite YC company, and it exists only by the blessing of cell carriers. (Though one could build an iPhone/Android Loopt today without anyone's permission, but I reckon you'd have tough competition, since I'm sure Loopt will be in that space as well.)

Couldn't loopt run on WLAN, too?

Well that could be it. But still i think - if idea is adding more value to current industry then there is a scope. But I am not PG ( not yet ;) ) so, my thinking doesn't count ( counts for me - not for others ).