It seems that u didnt heard the presentation at e3 did u? they said that all of the games on the 3DS will run in 3DS MODE there wont be a ds mode or anything all of the games including DS will run in 3DS regardless of wat they are. If u dont believe me go watch the Press of Nintendo again and u will see.
Other than "Compatibility with DSi", there was no mention of the presentation saying that DS games would run under 3DS mode (and if it does say it, please tell us the time index so we can confirm it), but as far as "compatibility" is concerned, that IS the DS/DSLite/DSi mode. Besides, running DS games under 3DS mode is not possible outside of emulation because of the hardware differences between the systems, and the system has to "change" how its hardware operates in order to achieve complete backwards compatibility, just like how the DS/DSLite changes to GBA mode for GBA games. Under these compatibility modes, any hardware not native to it (such as the case with the 3DS, being the 3D effect) cannot be used, and is literally shut off and inaccessible.
This is from the 3DS press release pdf:
3DS PR