We really would like to think this is inevitable, but it's just been sourced from Nintendo itself (albeit at a GameStop meetup) that the DSi
might be getting Game Boy and Game Boy Advance titles as downloadable DSiWare
-- in addition to previously announced apps like browsers and calculators. This was heard second-hand from a Club Nintendo member, and Nintendo has pointed out to the folks at Kombo
that reported it that so far such functionality has only been announced for Japan, but didn't go on the deny it outright. If it does come true we're supposed to expect some first party titles out of the gate, with all games being region locked and ranging from $5 to $8. There were also mumblings of playing downloaded titles off of SD cards, which Nintendo shot down as well, but hopefully we'll at least be able to make backups -- we're still waiting on similar functionality
for the Wii.