The DS doesn't appear to be much more powerful than the N64 in the first place, so I'd imagine a full-speed emulator would be unlikely. There's also the issue of controls.
If the DS isn't capable of running N64 emulators, it sure can handle graphics of a similar level - as seen in Mario 64 and particularly the new version of Snow Board Kids? Presumably these weren't direct ports of the originals but newly-created games? Either way, Nintendo seems to be relying on its past quite a bit when it comes to the DS, though with such a high quality track record I shouldn't really complain!
DS doing n64 might look considerably ugly and in my opinion too slow, to be any good. SM64 wasn't a direct port from the n64 game.it was rebuilt from scratch, using some common components (art wise, and gameplay wise).
also from what I've heard the DS contrary to the N64 got no texture filtering.. something I think is really bad =/
Ds is not strong enough to emulate the N64. Don't forget the n64 had a floating point unit which is critical in most games. Ds the other hand has zero floating point unit. Emulating a float on a arm9 is very slow.