I will flesh this out with comments later. Suffice to say, if one needs to mirror the applications installed on one Debian-based linux distro (such as Ubuntu) to another (also Debian-based, preferably running the save release) one merely needs to generate a list of those apps installed on host A and configure host B to install the same apps. This is done using dpkg and apt-get.
Generate the list by logging into host A:
# dpkg --get-selections \* > apps.lst
Copy apps.list to host B and execute the following:
# dpkg --set-selections < apps.lst
# apt-get -u dselect-upgrade
If there are apps which need to be installed you will see the familiar apt-get/aptitude interface displaying the apps to be installed and a prompt to confirm. Once confirmed, the apps will be installed and both machines should be mirrors (from an application standpoint) of each other.
Brain Dump 