For reference, the Japanese version sold about half a million copies on the DS and another half million on the PS3. That's higher than Xenoblade or The Last Story even managed in Japan.
Now, in the US I think it may be a harder sell and I don't really see a big marketing push for it or even a decent word-of-mouth campaign like Xenoblade had. On the other hand, it's also available at several different retailers. If they'd put a bit more into marketing it to kids (TV ads during children's programming blocks, etc...), like it was in Japan, it could really do some good business I think.
My own personal estimate is somewhere around 130,000 units, but I would love to be wrong and see it sell more.