The DSi is out in Japan real soon. But as far as the US and Europe are concerned? Nintendo Europe say "Spring 2009", Nintendo of America say "late 2009". But what about Nintendo Japan? They're the boss, what do they say? Nintendo boss Satoru Iwata says this:
Specific plans will be unveiled by our local subsidiaries, but an autumn or year-end launch would probably be too late.
If autumn or winter are too late, then it'll have to be out in the summer. Or maybe even the spring, if you're feeling optimistic. Nintendo says sees overseas DSi launch by next summer [Reuters]