Once read about how its a bad idea to base the core of your business around someone else's platform. Once they(and their lawyers) decide to re-evaluate your relationship with them it can nearly destroy what you've built. Testament to this are the number of apps based on twitter that died when twitter decided to up their offering and cover what they were doing. How to do it well is Zynga's relationship with Facebook

The people at PadMapper are doing a good thing looking for other sources but like the author said Craiglist was/is an important source of pad listings.

All the best to them