One thing that doesn't seem to be present in Ubuntu 10.04 is the little notification icon that shows up on your panel when the update manager has found programs to install. If there is some notification I haven't seen it.
Apparently this has been going on since 9.04, but for that version of Ubuntu I was running IceWM rather than Gnome as my window manager and so missed it. But Watching the Net didn't, and shows the way to restore the Update Icon by turning off the update notifier, whatever that is.
Apparently once the new improved update notifier (which I've never seen in action) is turned off the old style notifier kicks in. This is in the province of gconf, that all-seeing, all-knowing application that tells other programs what your likes, wants, and needs are — sort of like Facebook.
So to turn off the new-improved version and go back to the thing that works, open a terminal window and type (or cut and paste):
gconftool -s --type bool /apps/update-notifier/auto_launch false
Alternatively, launch gconf-editor, click on the + sign next to apps, scroll down to find update-notifier, click on that, and then uncheck the box that says auto_launch.
Either way, you're done, and the old icon will come up the next time you have an update available.