I concur... Today my update-manager (version 0.42.2ubuntu22 on Dapper) shows 23 updates available. None of them has a changelog displayed. The "Change" field briefly displays "Downloading the list of changes..." and then stays blank when clicking on each update.

The incriminated packages are: dpkg dpkg-dev dselect firefox firefox-dom-inspector firefox-gnome-support hal hal-device-manager info libavahi-client3 libavahi-common-data libavahi-common3 libavahi-glib1 libhal-storage1 libhal1 libnspr4 libnss3 libpng12-0 libpng12-dev mozilla-thunderbird mozilla-thunderbird-inspector mozilla-thunderbird-typeaheadfind texinfo

Aptitude is able to show the changelog for some of them but not all:

$ aptitude changelog dpkg dpkg-dev dselect firefox firefox-dom-inspector firefox-gnome-support hal hal-device-manager info libavahi-client3 libavahi-common-data libavahi-common3 libavahi-glib1 libhal-storage1 libhal1 libnspr4 libnss3 libpng12-0 libpng12-dev mozilla-thunderbird mozilla-thunderbird-inspector mozilla-thunderbird-typeaheadfind texinfo
Get:1 ChangeLog of dpkg [197kB]
Fetched 197kB in 0s (302kB/s)
Get:1 ChangeLog of dpkg-dev [197kB]
Fetched 197kB in 0s (287kB/s)
Get:1 ChangeLog of dselect [197kB]
Fetched 197kB in 0s (259kB/s)
Get:1 ChangeLog of firefox [128kB]
Fetched 128kB in 0s (254kB/s)
Ign ChangeLog of firefox-dom-inspector
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/universe/f/firefox/firefox_1.5.dfsg+1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for firefox-dom-inspector
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/f/firefox/firefox_1.5.dfsg+1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for firefox-gnome-support
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/h/hal/hal_0.5.7-1ubuntu18.2/changelog
E: Couldn't find a changelog for hal
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/h/hal/hal_0.5.7-1ubuntu18.2/changelog
E: Couldn't find a changelog for hal-device-manager
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/t/texinfo/texinfo_4.8-4ubuntu0.1/changelog
E: Couldn't find a changelog for info
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/a/avahi/avahi_0.6.10-0ubuntu3.2/changelog
E: Couldn't find a changelog for libavahi-client3
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/a/avahi/avahi_0.6.10-0ubuntu3.2/changelog
E: Couldn't find a changelog for libavahi-common-data
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/a/avahi/avahi_0.6.10-0ubuntu3.2/changelog
E: Couldn't find a changelog for libavahi-common3
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/a/avahi/avahi_0.6.10-0ubuntu3.2/changelog
E: Couldn't find a changelog for libavahi-glib1
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/h/hal/hal_0.5.7-1ubuntu18.2/changelog
E: Couldn't find a changelog for libhal-storage1
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/h/hal/hal_0.5.7-1ubuntu18.2/changelog
E: Couldn't find a changelog for libhal1
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/f/firefox/firefox_1.firefox1.5.dfsg+1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for libnspr4
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/f/firefox/firefox_1.firefox1.5.dfsg+1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for libnss3
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/libp/libpng/libpng_1.2.8rel-5ubuntu0.1/changelog
E: Couldn't find a changelog for libpng12-0
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/libp/libpng/libpng_1.2.8rel-5ubuntu0.1/changelog
E: Couldn't find a changelog for libpng12-dev
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/m/mozilla-thunderbird/mozilla-thunderbird_1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for mozilla-thunderbird
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/m/mozilla-thunderbird/mozilla-thunderbird_1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for mozilla-thunderbird-inspector
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/m/mozilla-thunderbird/mozilla-thunderbird_1.5.0.8-0ubuntu0.6.06/changelog
E: Couldn't find a changelog for mozilla-thunderbird-typeaheadfind
E: Couldn't fetch URL http://changelogs.ubuntu.com/changelogs/pool/main/t/texinfo/texinfo_4.8-4ubuntu0.1/changelog
E: Couldn't find a changelog for texinfo

And I did wait at least one day before trying to apply those updates.