I'd imagine that not many doubted this, but in case you were wondering it appears that my Japanese 3DS plays US DS releases just fine. The above copy of Shin Megami Tensei: Strange Journey (awesome game) plays without issue. So does a couple of my European DS games. That's probably because it wasn't until the DSi age that they started tacking on region protection. For example, my US copy of US Korg DS-10+ does not boot in my Japanese DSi. Or this system.
Fun note: The DS game icon appears in 3D on the top screen when you pop in a DS cart.
Back to region protection: So, to flip it to your situation, when you get your 3DS, feel free to nab any import DS games from Japan or Europe. They'll likely all work. As for the 3DS titles, we'll have to wait until I can get ahold of a US release. It' be a month or so, unfortunately. And I'm not very hopeful that they'll work.