I had little trouble with NetworkManager on Ubuntu 8.10. Even after applying the fixes mentioned on launchpad and forums etc, I was unable to get it work. But last week I successfully managed to get it working on Jaunty (Ubuntu 9.04 Alpha 3) upgraded from its earlier version Intrepid Ibex.
I made a small change in as mentioned below in /etc/NetworkManager/nm-system-settings.conf, to get NetworkManager working after upgrade.
# cat /etc/NetworkManager/nm-system-settings.conf
[main]
plugins=ifupdown,keyfile[ifupdown]
managed=true
Once you confirm the settings, restart the NetworkManager.