Will an iPod touch suffice for testing iOS apps? I am buying a testing device for iPhone apps. I cannot afford an iPhone 4 so I am thinking to buy an iPod touch. Can I use an iPod touch for testing iPhone apps? What will be impossible to test on an iPod touch?

My second thought is to buy an iPhone 3G or iPhone 3GS, but I am not sure if these devices are too old to use as testing devices. What do you think? Could I use them? Can a new system be installed on these devices (does it slow them down)?
I wrote an extensive blog post about this topic a while ago: http://wjlafrance.net/?p=188

Basically, an iPod touch is great as long as you don't want to use GPS-based location in your app (you can test general, wifi-based location) and are only interested in iOS 4+ deployment (or iOS 5+ if that's what your iPod is running).