It's pretty cool because it doesn't require any manual intervention other than specifying the location and giving it a name. iOs has a feature called region monitoring that fires an event when you arrive at and depart from an area. Region monitoring was introduced earlier, but it has been improved for iOs 5 and and is now practical for detecting location accuracy to 100 meters. Strangely, it works on all iOs devices, even those lacking a gps, like an ipod. The reason is it primarily detects it's location based on the wifi signals that it detects.
The app is 5 screens and I estimate the whole process took about 100 hours. I ran a 99 designs contest and I paid $295 for an app icon and a splash screen. I also had to pay $99 to join apple iOs developer program for a year. I am planning to charge $.99. Apple takes %30 of the revenue. However, I think it's a good deal. They handle all the issues associated with collecting payments, like fraud, refunds, sales tax, and the fees and hassle of dealing with payment processors. They also host the store and I don't have any server side components, so I am off the hook regarding hosting maintenance or scalability issues.
I am optimistic about OnSite because it's a cutting edge app. It wasn't pracitcal to do this on an iPhone before iOs 5. My primary challenge now will be to market the app.
I just submitted the app and it is in the approval process. It should be available soon. iOs 5 is available on Wednesday October 12th.