Let's look at the Apple license terms for a second. These sections come from the app store terms of usage that are copied on the FSF website, not the iTunes terms of usage. FSF says that the analysis applies identically to both, but they seem quite different to me. Anyway, the interesting bits from section 9:
(ii) You shall be able to store Products from up to five different Accounts on certain iPhone OS-based devices (including, but not limited to, iPad, iPod touch or iPhone), at a time.
(iii) You shall be able to store Products on five iTunes-authorized devices at any time.
The storing part could be seen as a limitation in copying, but I think it is not. What happens if you register a sixth device is that Apple will simply not distribute the program to that device since you already have five. Denying to distribute something to someone is, as far as I know, not a violation of the GPL license, since the license only comes into play once you distribute something. No distribution equals no license problem. Of course, Apple should make sure that the other five copies are distributed GPL compliant.