I had that problem with boot hanged on Timidity+Alsa emulation after a regular update through apt-get. I tried to figure out what was the faulty package but could not.
I initially tried to relaunch graphical interface through sudo restart lightdm. I was not successful in that but in /var/log/lightdm I found lightdm.log and x-0.log which were helpful to diagnose.
In the last one, I could find warnings (WW) about fglrx which is the main process for ATI drivers.
It was talked about 'unallocated seat' and other stuff making me think the driver was a problem.
I uninstalled my ATI driver through sudo aticonfig --uninstall then rebooted the computer.
Graphical interface was back and I just had to reinstall the driver with sudo sh ./ati-xxxxx.run
Hope this will help
Ubuntu 12.04 through an Ultimate distribution (3.4)
Gigabyte 990XA-UD3 / Phenom II X4 / 8 Go Ripjaws / Sapphire 7870 2Go
... and many hard drives... :)