Code Sign error: The identity 'iPhone Developer' doesn't match any valid certificate/private key pair in the default keychain

This happens if you forgot to change your build settings to Simulator. Unless you want to build to a device, in which case you should see the other answers.

Check if you are building for device instead of simulator. Go to Xcode menu 'Project' -> 'Set Active SDK' change from 'Device' to 'Simulator'

Under Xcode 4.1 Check your build settings for the project and your targets. For each check under 'Code Signing' check 'Code Signing Identity' and change over to 'Don't Code Sign'

This usually happens to me when I forget to change the company to match mine.

1. Select the target under Groups & Files
2. Click the Get Info button
3. Choose the Properties tab
4. Under Identifier, make sure it says com.YOURREALCOMPANYNAME.APPNAME

When you create a new app, it usually says, "com.yourcompany". Change it to whatever you registered with, in my case com.DavidKanarek

if you are just using the simulator and just upgraded then this solved the issue for me: go to menu->project-edit project setting. find code signing section (you can type 'code' in the quick search) in the code signing identity select 'any sdk' and set the value to 'Don't Code Sign'