Sometimes thing's come out almost at the same time, maybe it's cause fpgas and hardware has become much cheaper in recent years.
Dknute is working on a Dreamcast SD adapter that will be far superior to the halfassed chinese SD adapter from past year that played games choppily, so finally there will be a real replacement for the gd-drive. You can follow his progress here: http://dknute.livejournal.com/
And then the PSIO team is working on a SD adapter for the good ol' Psx that you can follow here: http://psio.com.au
Let's not forget that the russians made a adapter for the 3DO half a year ago:
So now it's almost just the Sega Saturn left! (and PCFX,CD32,Fmtowns,PcEngine,JagCD,NGCD,Pippin,Megacd,Cdi)