It seems that there are no Processor_MHz_GHz, CPU_core_count, or GPU_shader_count handlers that are currently registered for the UIRequiredDeviceCapabilities plist. Currently, Apple does not allow adding additional hardware restriction keys to existing applications already in the application store.
So, it seems to you that you are out of luck if you do not want to take risks, demanding the camera. Or simply selling a new application (rather than an update) to support hypothetical future products.
ADDED/UPDATE (2012-): iPad , iOS 6.x , iOS 6 Apple iPad, iPad 2 iOS 6.