I just switched from Xcode 3 to 4. When I attempt to upload an app that I have archived to the organizer, I receive this error:
The archive is invalid
The same happened to me. My mistake was I had certificates with similar names (like "Provisioning Profile of Giammy", "Profile for Provisioning of Giammy"...).
Running "Archive" from XCode 4.0 and then "Validate..." from the Organizer I faced the "app.ipa does not exist" issue. The problem was that I selected the wrong certificate from those with similar names. The "Archive" phase worked good but the archive did not pass the validation.
Solution: just picked up the correct provisioning profile in the "Archive" phase.
Lesson learned: check twice the provisioning profile name!