I've had a similar problem and it seems to be related to Network Manager. To resolve it I booted into the recovery mode (selected when you first switch on your machine) and selected the root prompt. At that point I removed Network Manager
which will remove network-manager, network-manager-gnome and possibly network-manager-kde.
apt-get remove network-manager
A quick word of warning here, I don't know whether removing this will disable your network access, so you should ensure that you have the means of reinstalling this before you continue, ie a live CD of a known working release of Ubuntu. If you do lose network access I have posted instructions to get it back which work for me below.
At this point I typed "exit" to get back to the screen which will allow you to resume the normal boot process, now all works well.
As mentioned above, once you get to the desktop you may not have network access, open a terminal and type
and look for the name of your network interface which will be something like eth0 or wlan0.
To enable the network type
replacing <network interface> with the name of your interface that you found in the previous step.
sudo ifup <network interface>