Speaking to Pocket Gamer, an obviously-doesn't-want-to-be-named "developer" has said there may be a problem or two regarding DS/DSi compatibility. This developer, you see, is currently developing a game for the DS. Their game obviously runs just fine on DS, but on the DSi, it keeps freezing up, and as a result is stuck in development limbo, Nintendo refusing to certify it. Nintendo, predictably, are saying "All DS titles are compatible with DSi", but if a game in development will run on DS but not DSi, might there be some finished games that run into problems as well?
Developers facing compatibility issues with DSi? [Pocket Gamer]