Well, it"s been a little while since the XBox was released and people are already playing online! The folks at xboxgw.com have written a program to create an ethernet bridge between xbox"s over the net, fooling the xbox"s into thinking they"re on a LAN. 1v1 seems to create no major lag and reports have come in like this one that claim up to four xbox"s work fine.
*I Might try this myself with my XBOX, plus there will be a review of the XBOX by me soon*