This also worked for both of my iPhones (5 & 5s) however in my case the app doesn't have to be open in the background...
Go to Settings - General - Reset
Press "Reset Location & Privacy"
Establish a Location based reminder using Siri
Siri will tell you that you need to enable reminders in "Settings - Privacy - Location Services"
Once Reminders is enabled return to Siri and establish a Location based reminder. You should see the hollow arrow geofence graphic in top information bar. Your reminder should fire as in ios 6 now.
Over several days of setting reminders to alert when "arriving/departing home or work" the alerts have become more precise...meaning instead of alerting within a half mile of destination/departure point the reminder alerts more like 3 to 500 ft. of the destination/departure point.
IF you open the reminder app you will have to repeat the above process. I've put the reminder app in a folder with other system apps I don't use and pushed it to my back page. I just don't use the reminder app now.