Nintendo wasn't present at the show itself (only a keynote), so I don't know if there were DS demo's at all...
apparently sega had some, IGN reported on it http://ds.ign.com/articles/651/651827p1.html I'm sure someone will have them uploaded eventually ^_^
Yep, Sega had about 5 DS demos for download including Puyo Puyo Fever 2 and Where do babies come from. Never tried them myself but I did see an awful lot of people with them. Let's just pray that someone will upload them to the net. Yakumo