Also like mentioned already, if it plays the old games, what is the difference?
I'm with Ian on this one. The difference is that Nintendo would start making new games which wouldn't work on the current DS, and that would be a kick in the teeth for those who just recently purchased a DS.
Also, if Nintendo was planning on releasing a new handheld in 2010, then they need to wait until E3 of 2010 to announce it, because if they announce it now Sony will have plenty of time to start ripping off whatever features they announce for it, and consumers would also refuse to buy the DS, because they are aware something more capable is on the horizon.
IMHO, it would make much more sense for Nintendo to come out with a new hardware revision for the DS, rather than a successor. I'm sure they could find a way to make it smaller, have some built in storage capacity for VC games, rumble support, MP3 playback, et cetera... None of the games for it should be incompatible with current DS models, though.