Checked source.list.d files and source.list file that you specified. I dont think I have http://ppa... or anything link with word boot enabled.  But, still your solution worked once and then, I tried to boot in safe mode and repaired the packages. Again, I'm at the same point. unable to boot :(

Next, I chrooted and did 'sudo apt-get update'  - thinking that this might fix (i think it created a mess)

and tried to follow the step
"After that, you can downgrade:
sudo aptitude install mountall/karmic"

The o/p is below:
root@ubuntu:/# aptitude install mountall/karmic

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Reading extended state information      
Initializing package states... Done
The following partially installed packages will be configured:
  rsyslog ubuntu-minimal virtualbox-ose-source 
0 packages upgraded, 0 newly installed, 0 to remove and 5 not upgraded.
Need to get 0B of archives. After unpacking 0B will be used.
Writing extended state information... Done
Setting up rsyslog (4.2.0-2ubuntu5) ...
start: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
invoke-rc.d: initscript rsyslog, action "restart" failed.
dpkg: error processing rsyslog (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of ubuntu-minimal:
 ubuntu-minimal depends on rsyslog; however:
  Package rsyslog is not configured yet.
dpkg: error processing ubuntu-minimal (--configure):
 dependency problems - leaving unconfigured
Setting up virtualbox-ose-source (3.0.8-dfsg-1ubuntu1) ...
No apport report written because the error message indicates its a followup error from a previous failure.
                                                                                                          Adding modules to DKMS build system
Doing initial module builds

Error! Your kernel source for kernel 2.6.27-7-generic cannot be found at
/lib/modules/2.6.27-7-generic/build or /lib/modules/2.6.27-7-generic/source.
dpkg: error processing virtualbox-ose-source (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 rsyslog
 ubuntu-minimal
 virtualbox-ose-source
E: Sub-process /usr/bin/dpkg returned an error code (1)
A package failed to install.  Trying to recover:
Setting up virtualbox-ose-source (3.0.8-dfsg-1ubuntu1) ...
Adding modules to DKMS build system
Doing initial module builds

Error! Your kernel source for kernel 2.6.27-7-generic cannot be found at
/lib/modules/2.6.27-7-generic/build or /lib/modules/2.6.27-7-generic/source.
dpkg: error processing virtualbox-ose-source (--configure):
 subprocess installed post-installation script returned error exit status 1
Setting up rsyslog (4.2.0-2ubuntu5) ...
start: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
invoke-rc.d: initscript rsyslog, action "restart" failed.
dpkg: error processing rsyslog (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of ubuntu-minimal:
 ubuntu-minimal depends on rsyslog; however:
  Package rsyslog is not configured yet.
dpkg: error processing ubuntu-minimal (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 virtualbox-ose-source
 rsyslog
 ubuntu-minimal
Reading package lists... Done             
Building dependency tree       
Reading state information... Done
Reading extended state information      
Initializing package states... Done