From what I can interpret from the runtime requirements of sample region monitoring code of Apple, region monitoring is available on iPhone 4, iPad 2 Wifi + 3G or later. So I'm assuming that it's not available on iPad1 with 3G, or iPhone 3G/3GS.
Is this a correct interpretation? I cannot test this since I don't have all those devices and I couldn't find an official documentation about this.
PS: I know that I can check for it in the code by calling regionMonitoringAvailable
method but I need this information to decide for the architecture of my app, so I need to know this upfront.