There was an old bug reported on update-manager about it's lack of support for non official repositories change logs display. This was something already requested on getdeb and it benefits most people which use PPAs, checking the change logs usually provides a good hint on the upgrade risk.
I had proposed a possible solution in the past, today I have check with Michael Vogt that the proposal was ok. I got positive feedback so this was the time to help with some code.
I don't have much experience with python-apt so the research over the weekend using winpdb (the python debuger) was fundamental. I have found the function responsible for the changelog file retrieval, get_changelog() at UpdateManager/Core/MyCache.py and added the change log URI construction based on the package origin server, later Michael fixed it using the ArchiveURI from the package.
The patch introduced a new feature so it required a Feature Freeze approval, it was granted quickly, it is a small patch providing a perceived benefit.
Please note this feature depends on the changelog files being available on the server, at ARCHIVE_URI/changelogs/... (pool structure). The repository maintainers need to implement their own changelog extraction utilities. GetDeb has already implemented it, however it will only be available once we upgrade our build server to 10.04 (there are some other building related changes blocking).
A task was also submitted to Soyuz so PPA's should support it in the future.