I have been using the new \'Build and Archive\' feature of the latest Xcode 3.2.3. I like it.
Now I noticed that it is always disabled for some reason. I can\'t se
in my case, the archive checkbox was unchecked. =/ strange

Had been messing around with this for quite some time too. Don't forget to click on 'Validate Settings' at the bottom after you've checked the 'Prerendered' tickbox in the summary tab. And if you're deploying your app to the iPhone or iPad, make sure you delete the app first and then build it again. Otherwise your icons will still be the same.
in xcode 10.2:
make your project on Generic ios devices tab and go to product>archiveto archive your project after if you want to see all your archived history go to windows>organizertap .

Build configiuration setting needs to be an "iOS Device" (or any specific device if connected) to activate "Product → Archive" menu item. For some strange reason when any kind of iOS Simulator is selected, Archive is greyed out.