I have just updated my iPhone to iOS4 today and lost the PolyTune app in the process. Is it possible to get it back?
During the iOS4 update iTunes should automatically have made a backup, so if you sync your iPhone you should be able to transfer it back to your phone.
Alternatively, since you have already downloaded it once (either for free or purchased), you should be able to download it again from the app store for free.
If not, you would seem to have an issue with your iTunes account, in which case you should get in touch with Apple for a resolution.