Versions of Ubuntu that are not yet released (pre-alpha/alpha/beta/rc) don't seem to notify when updates are available. Stable releases notify me on occasion until I dismiss them and then come back, but I can't figure out when.
How does the update manager determine when to prompt users?
Update manager is configured not to notify on updates on pre-release versions of ubuntu. This prevents update notifications from occurring all day long due to frequent updates during the pre-alpha through beta phases of a release cycle.
It's important to manually update frequently or you won't be able to report issues due to out of date packages.