Notice how *fast* his hands move when flicking the switch? Notice how the hands actually look more like stop-motion animation rather than real movement? Heck, notice the artifacts on the screen moving faster in 'overclock' mode? Now how on Earth would the artifacts introduced while recording the video move faster? That's some 'overclock'!
Anyone can record a video while moving their hands really slow, and then speed it up to make everything look like it's going faster. The performance of Mario Kart alone makes the 'overclock' very skeptical. Anyone who has ever really overclocked anything would be raising an eyebrow as to how everything stays remarkably in-sync without a single timing issue.
Furre managed to overclock the DS authentically quite some time ago, but when going online, the clock gets reset to it's normal value. This particular video is highly suspect, and careful examination brings up way too many questions for me to be comfortable.
I don't want to sound like/act like a jerk, but I'm locking the topic because I wouldn't want someone to simply believe this to be true and ruin their DS Lite. If someone here actually manages to do it, I'll eat my hat and re-open for discussion, but there's just no way that video is an authentic recording of an authentic overclock.