I get this result on my newly installed ubuntu 9.04 netbook remix. I haven't tampered with any apt files at all. It doesnt work for any other packages.
ira@ira-windu100:/etc/apt$ sudo apt-get install git.core [sudo] password for ira: Reading package lists... Done Building dependency tree Reading state information... Done E: Couldn't find package git.core ira@ira-windu100:/etc/apt$