JMS said something about this when S1 was just going into production. I don't have the post and I'm not sure I remember the details exactly.
The gist of the message was that there is an embargo on both syndication and home video. I think Showtime gets to run each season in its entirety a couple of times before that season can be syndicated, and that season has to run in syndication for a year before it can be released on home video. If I remember the math they were talking about the first season hitting broadcast TV/non-premium cable (or at least being made available to them - nobody can say for sure that anyone will
buy the show

) in 2004, which means the earliest date for S1 on DVD would be 2005.
But, hey, that's just what I remember. I could be wrong.
Regards,
Joe