Thought this might be of intrest, if anyone has further information please send it along.
I would LOVE to have modem/PPP support for dreamcast.

http://boob.co.uk/cgi-bin/ikonboard3/ikonboard.cgi?s=414b7cf12d9affff;act=ST;f=4;t=1195;st=0

http://www.boob.co.uk/devtools.html <-- See Modem section, partial drivers written.