According to the "Big Three", 358/2 will be out on May30th for Japan. So, for North American, that would probably mean November or December(right on time for the Holidays, as usual).
Some other Non-KH games will be coming out in May too, but obviously, I won't be saying much about those here.
I expect the KH fandom on Youtube to fire up their videos in late May/early June because some of them HAVE TO HAVE IT RIGHT NOWWWWWW!!!! Plus, they can read/understand Japanese, and will try to be the first to report it. People who run the "Big Three" sites will be among the first to report results, of course. Won't be along now. The end of May will be here before anyone knows it.
I just hope that 358/2 works for the regular DS Lite. I'm pretty sure it does, but with the DSi coming out, I do doubt a little bit. However, the DS Lite is still going strong, so the doubt isn't all that great.