If you do transition to iCloud sync in the future, will you keep the Dropbox functionality? I ask for two reasons; the non App Store version won't have access to iCloud (unless something changes by then), and I need to use a shared Dropbox folder to share OLLY data with my wife (and I don't see the ability to share data like that between iCloud accounts coming any time soon).
For less potential trouble with Dropbox sync, you might look into what I've seen some other developers do to avoid conflicts. One approach is to use a temporary file to declare "in use" status to other clients that try to connect, dropping them into a read only mode. Another (more complicated) method is one that I don't have such a simple explanation for, but it is used by No Thirst Software's MoneyWell. Kevin Hoctor seems like a good guy, and he might be willing to share the basic concepts. All I know is that I keep MoneyWell open on my Mac, never think twice about entering data in the iPhone, and sync happens automatically.