According to a secondhand report from one of last night's GameStop Club Nintendo DSi preview events, Nintendo is planning to release Game Boy and Game Boy Advance games as downloads in the DSi Shop. According to Kombo's "man on the scene," only first-party Nintendo titles will be offered at first.
Other details from the event: Apparently, North American DSiWare includes three pricing levels: free, 500 Points, and 800 Points, which differs from the Japanese model in that there's no 200 Point category. The event seemingly confirms the Mario and Animal Crossing calculators for North America as well, though both of those are in the 200 Point category in Japan.
Of course, like all news that comes from something heard at GameStop, take this with as much salt as you can find.