I'm needing to purchase whatever devices are needed to do testing for iOS applications. My initial thoughts are to test on a first-generation iPod Touch and an iPhone 4. Testing on the old iPod Touch with 3.1 software will give a good indication on how the app will work with little memory, and the iPhone 4 will obviously be a test for the faster system.
What do you all typically test on before submitting an application?
right now, I test using a 3g running 3.1 (a popular build), a 3gs running 4.0.2 (latest), a touch running 3.1.3(basically 3.1), and an iPhone 4 with 4.0.2.
Mainly try to keep a 3.x device around for sure, and then obviously a device with the most up to date OS.
Based on AdMob reports...around 98% of users are upgraded to atleast 3.0 and those that aren't shouldn't be your client base anyway (unlikely to use new apps).
At minimum, a device running the oldest OS that I list as my Deployment Target, the slowest device/OS combo that I have (e.g. a 3G running 4.0.x is likely slower than anything running a 3.x OS) and which the app supports, and the current/latest released OS.
A device which includes all the features that my app will support (e.g. mic, camera, gyro, GPS, etc., if needed.)
Optionally, an iPad if the app is Universal, or if not to see how bad 2X zoom looks.
Optionally, a device with a beta OS installed if it appears Apple is going to release it well before I plan on revising the app.
So far all the apps I've submitted have been compatible with iOS 3.1.x, and I test them on my old 1st gen iPod Touch. I also test on my 3.2.2 iPad and my 4.0.2 iPhone 3GS.
My guess is all of these answers are going to be along the same lines -- test with as wide of a variety of devices as you can that are compatible with your app (i.e. meet the Minimum Deployment Version).
This is especially important if you develop Universal Apps that support 3.1.x. You'll find a ton of bugs with the hardware that you will never get with the simulator.