> No...  You will not be able to play Sonic Adventure on line...  But, you will be able to use your Uberconsole's modem to access the Sonic Adventure Homepage.  Doing so will allow you to get hints, tips, participate in a time attack world ranking area, and trade Chao creatures.  There will be additional features for owners of the game, but details have not been divulged as yet.  One possibility would be downloading of various patches to the game that allow for new graphical themes (this was already done in Japan).  
(source: Next Generation by: Shinobi)

     > Sega has reportedly constructed an experimental test server for the forthcoming Dreamcast translation of Nextech's Dark Eyes.  Dark Eyes is essentially an online RPG which will be ported from the Windows version.  According to the source, the new server features integrated 'party system' technology.  This should allow players to link up into groups, with the server retaining all relevant gameplay statistics and data.  A confirmed release date for 
Dark Eyes on the Dreamcast hasn't been disclosed, but it's supposed to reach Japan in late 1999.
(source: Core by: Shinobi)