Last friday Mina Shokry joined in#Ubuntu+1 reporting an unbootable system after upgrading to Lucid. Even booting with rescue mode it was hanging without any error message, apparently it was after mounting the file systems (guessing by the "mounted filesystem" message).
After several hours trying all sort of boot options we are able to identify the problem.
The /proc/bus/usb on fstab which was previously required for USB support in VirtualBox mount was failing, however upstart/mountall (not sure where is the fault) did not provide an error and simply stopped the boot process.
The bug for the mount issue is bug 565109 .
I have also filed a bug for upstart, bug 565890, simply making it verbose when using rescue would allow to easily identify the problematic task.
If you use VB's USB support make sure you remove the /proc/bus/usb entry from your fstab.