I was working on my iPhone app last night. It requires the features of iOS4. I found a weird behavior. When I ran my app in debug mode on a device, it ran properly. When I ran my app in release mode on a device, I was getting behavior like I was running on iOS3. I was pulling my hair out. After I got back from the gym, I listed my problem on the MonoTouch IRC. Thankfully, someone else had that problem. The issue is that there are different settings for Debug and Release with the Simulator and iPhone. As a result, you have to set the settings appropriately for each configuration. Thankfully, you just need to be aware of this and set it as necessary. Here's the screen you need to be aware of.