Important New information for Ubuntu Lucid and newer!
Starting with version 3.0.1-1 for Ubuntu there is a proper signed repository that can be added to your system.
The Synaptic Method:
1. In Firefox, go to :
http://www.remastersys.com/ubuntu/remastersys.gpg.key
save file as text someplace where you can find it.
2. In synaptic, go to Settings/Repositories; select "Authentication" tab and "Import Key File" just downloaded.
3. Still in synaptic, go to "Other Software" tab and click "Add", then enter the apt line and replace oneiric with either lucid, maverick, or natty to match your Ubuntu version:
deb
http://www.remastersys.com/ubuntu oneiric main
4. Leave the repositories tab and "Reload".
5. Search for "remastersys" and select for install. Edit/Apply Marked Changes.
NOTE: There is no source repository so if you see a line starting with deb-src you need to remove it.
The Manual Method
As root - issue 'sudo su' in the terminal window prior to the following command.
Download and apply the repository gpg key.
wget -O -
http://www.remastersys.com/ubuntu/remastersys.gpg.key | apt-key add -
Add the following line that corresponds to your version of Ubuntu to your /etc/apt/sources.list
#Remastersys Lucid
deb
http://www.remastersys.com/ubuntu lucid main
#Remastersys Maverick
deb
http://www.remastersys.com/ubuntu maverick main
#Remastersys Natty
deb
http://www.remastersys.com/ubuntu natty main
#Remastersys Oneiric
deb
http://www.remastersys.com/ubuntu oneiric main
#Remastersys Precise
deb
http://www.remastersys.com/ubuntu precise main
Now just apt-get update or reload in Synaptic to have the new Remastersys signed repository ready to use!