The Ubuntu kernel changelogs can be found in many packages across ubuntuupdates.org website, but the easiest place to find them is under the package called linux. For each Ubuntu release, there are 3 kernel versions available, one in each repository: proposed, updates and security.
The full list of kernels with their versions for any release and any repository can be found on the linux package_meta page.
From there, the changelog is just one click away. Follow the link for the version you want and you get to the package page with all revisions and changelogs, one by one.
For example in Maverick security, the kernel is linux 2.6.35-27.48. The proposed repository is usually ahead of security and updates but as of 3/1/2011, the same version is in all 3.
Sometimes the security repository skips versions and therefore, the changelog for those intermediate versions is missing from the security linux package page. The kernel in Maverick security directly went from 2.6.35-25 to 2.6.35-27.48 and this latter version changelog appears very small. But in reality, a lot changed. Go to the Maverick proposed linux package page to find all the intermediate versions. The bulk of the changes are in 2.6.35-27.47 and 2.6.35-26.46.
Note that all of those logs are just Ubuntu kernel logs and they rarely include much information on what happened upstream. For the upstream changes, visit kernelnewbies.org or kernel.org.
One last thing: Ubuntu most recent LTS release, Lucid 10.04 has a PPA with all the newest backported kernels. It's called Kernel PPA and it includes the original maverick kernel 2.6.35-22.34 as well as the brand new 2.6.38 Natty kernel.
