I ran into a problem with avahi that appears to be relatively common. My version is that the wireless networking fails and Network Tools shows a network device called eth1:avahi which shows a few transmitted and received bytes (with errors) but no interface information. This device appears to have preempted the correct eth1 device. I killed this rogue device using
sudo avahi-autopid --kill eth1
then after a while wireless works again as the eth1 device shows a connection.
At the suggestion of omingo I have done a
sudo apt-get --purge install avahi-autoipd --reinstall
to make sure the latest version of avahi-autopid is in use.
Update: this doesn't seem to have helped. Another possible workaround seems to be to go to Wireless Connections Properties using the Network Settings tool and switch from DHCP to Zeroconf, click on OK, wait for the tool to finish resetting the connection, then go back into Properties and switch back to DHCP.
Update: this may actually be a problem with network-manager. I've made the fix to NetworkManager.conf for bug 40125. Did a suspend then restart and the wireless connection came up correctly which it has not done before.