I have had succes using Antamedia DHCP server and a the simple 1 to 1 ethernet cable to vnc into "NewIT" guiPlug installed on my sheevaplug. This is for my laptop running windows vista.
Ref GUI Plug http://www.newit.co.uk/forum/index.php/board,23.html
Now if I want to go to pure linux to linux it seems I have not gotton DHCP3-server
the (non GUI) linux equivalent to work on my linux laptop, so that I can get the sheeva plug an IP address which would allow me to ssh to it, via linux Putty ssh client or to use a vnc on linux to talk to my sheevaplug.
If you are using linux to ssh into the plug (not via the serial port, but by the ethernet port) please respond , perhaps you could assist me.