I can't figure this out and I thought that someone might run through the same thing.
I have Xcode 3.2.3 (Pre Release with OS 4 beta) and I started to create my application, after the final touches and everything worked ok, I changed the Simulator - 4.0 to Simulator - 3.1.3 (latest iPhone OS) and I could never start my app again :-(
Does anyone know what I should do?
I created a simple Screencast of the problem so everyone can see what I'm writing about.
Thank you for all the help.
You should ask on the Developer Forums, since OS 4 is in beta and under NDA.
Independent of the NDA, the key thing to understand is that the base SDK does not have anything to do with you "deployment target", which basically sets what platform you are deploying to.
Go to your project settings and search for "iPhone OS Deployment Target". You can set that to 3.1.3 at your leisure. Even if your base SDK is 4.0 beta, it will work.
In the same way, if you had Base SDK 3.1.2, and wanted to be able to check whether you're safe on 2.1, you could set the iPhone OS Deployment Target accordingly.
I wish I could find the URL that answered this question for me when I had the same concern - it makes a very clear distinction.