I am using remastersys from ubuntu 9.10. I regularly use remastersys to customize my ubuntu. I search for installing remastersys on my ubuntu machine 11.04 today morning. I went through official remastersys website. If you are having problem in installing from aptitude by adding following repos you need to get .deb package.
For Karmic, Lucid and Newer with grub2 – version 2.0.13-1 and up
deb http://www.geekconnection.org/remastersys/repository karmic/
I am also stuck in installing remastersys by the above source.Then i go to their sourgeforge page to get .deb package.
I successfully installed remastersys on my ubuntu 11.04 from the deb package.
Hope this step step might help you.