Today there was a new kernel update - to version 2.6.32-25. I ran the update manager and the updates installed fine, and let it reboot. Upon restarting, I went to a terminal and checked what kernel was running: the previous one still. For some reason, after performing a kernel update, my system does not update the Grub menu to load the new version. Whether this is by design or it's just a quirk of my installation, I don't know. But let this just be a heads-up for other users. The security fixes in the new kernel won't be of any use if you are still running the old one!
Fortunately, it is easily fixed by entering "sudo update-grub" at a command prompt. It will look through your system and add the new kernel to the startup menu. After a while, when the new kernel appears to be working OK, you can uninstall the previous version to save some space.