Upgrading iPhone/iPad to iOS 4.0
Update 2, July 2010
Apple have released iTunes 9.2.1. This update appears to address the backup/restore issues.
Ensure you have iTunes 9.2.1 before attempting the upgrade. Use Apple Software Update to check for updates.
Update, July 2010
Apple have released iOS 4.0.1. This update installs very easily over iPhones that already have 4.0.
However the original issue with installing iOS 4.0/4.01 over iOS 3.x seems to be in iTunes and we have not seen any resolution yet.
We are advising all clients not to accept the iOS 4.0 update for their iPhones.
There appear to be numerous problems associated with the backup / install / restore process required to put the new operating system on the device.
Most likely Apple will need to update iTunes to make the installation more reliable.
However, if you have successfully installed iOS 4.0, then this won't apply to you.
Installing iOS 4.0 differs from previous upgrades in that it first backs up the device, wipes the data, applies the new operating system firmware, then restores the data (reinstalling apps, re-synching music, calendars, contacts etc). Previous upgrades simply loaded the new firmware over the existing files.
The main issue is a failure during the restore phase (error -35), which leaves the phone with only the basic settings, and no apps or media. Furthermore, most options are turned off in iTunes, so it is necessary to re-enable the synching of calendars, contacts, media, podcasts etc. Because there are no apps, and the restore doesn't work, the app settings (passwords, options, scores etc) are gone.
Upon attaching the device, iTunes prompts that a restore was incomplete, and asks whether to cancel, continue the restore, or delete the backup. If you choose Continue, the restore fails again. From that point, when trying to sync the media, iTunes reports that certain media files cannot be synched because the files cannot be found.
The solution to this problem is to unplug the device, close iTunes, reopen iTunes, attach the device, and choose Cancel when prompted about the failed restore. iTunes will then synch the media properly (assuming the options to do so are enabled). This may take a very long time (several hours) if the device has many gigabytes of media files.
Also, by default iTunes will not synch apps. Upon enabling app synching, the apps will be reinstalled, but when operated on the device, start, then close immediately. To resolve this, install any free app from the AppStore, and sync the device. Then unplug the device, close iTunes, open iTunes, and attach the device again. Upon synching again, the apps should work properly.