I have installed the new 3.0 SDK, but when I start up Xcode, I don't see an option to build for "Device - iPhone OS 3.0" or "Simulator - iPhone OS 3.0"
The Xcode About box says I am running: 3.1.2
Is there something else I have to do?
If you are using an existing project you may have to tell your project to build to the 3.0 targets. I didn't have to do this for any of my projects but ...
Under the Deployment section of your project properties you should see:
iPhone OS Deployment Target - (change this to use iPhone OS 3.0)
You can also check that the Architectures section has the Base SDK set to 3.0.
This happens quite often when downloading Apple's sample code projects which are targeted for iOS 3.x but you've got iOS 4.x.
To show Simulator option go to Project Info window -> General tab -> Base SDK for All Configurations - choose Latest iOS.
If it doesn't appear straight away, close and reopen Xcode.