I made sure to check out the day one supplies of the 3DS in my area, and there were plenty. Now, a week later there are still plenty. Just from some asymmetrical shelving arrangements, I'm guessing that a few have been sold in that week. Nintendo has reported that the 3DS was the best selling handheld launch in the U.S. in their history, but I'm not sure that really means huge numbers. It's a shame for the moment because I've only had one wild StreetPass encounter, and even that was from a GameStop employee (she had a very accurate Mii).
I'm not currently worried about the 3DS not being a success, and thus not getting great software support, personally. As seems to be Nintendo's plan, the 3DS will need to sell itself when people show it off to friends and family, which isn't going to happen instantly. It might need a few hard kicks from the likes of Mario Kart to really get that viral spreading started, like the first DS needed Nintendogs and Brain Age to start it on the path of the best selling video game console ever (has it reached that just yet?).