It is certainly a niche title, but one that I picked up while over there was DS Rakubiki Jiten (if I'm remembering the name right). It's an electronic dictionary for the DS, and it has built in English > Japanese and Japanese > English functions. The interface is wonderful, and it has some nice features, but there is no Kanji search support, which sucks, is obviously made for a Japanese speaker, and the extra options aren't as good as they could have been. It is a really handy tool if you just need an electronic dictionary for conversational needs, but otherwise won't replace the fancier electronic dictionaries.
My biggest recommendation would be, as others have stated, Ouendan. I got that and Band Brothers, but BB so far isn't overly impressing me. (I know some people just adore it though.) I think beyond those, Jump Superstars (I'm not up on if it is hitting our shores or not), and what is known to be coming over here, there isn't too much of high quality that is still stuck in Japan. I went on sort of a DS buying spree while over here, but to me it seemed like we were getting most of the good stuff already. I think by far Ouendan is the big pick for import titles that have little chance of making it over here.