See here:http://www.newit.co.uk/forum/index.php/topic,44.0.htmlAnd here:http://plugcomputer.org/plugwiki/index.php/New_Plugger_How_To#Fixing_the_existing_Ubuntu_install
I provided the links so that you can fix networking. This is unlikely to be an apt-get issue; it is most likely a networking issue. There are fixes required of the ex-factory sheevaplug.So:Is networking working?does ports.ubuntu.com resolve?
Try http://old-releases.ubuntu.com/ubuntu/ instead of http://ports.ubuntu.com/ in your /etc/apt/sources.list