Happy to answer both questions - background sync and iCloud support are kind of two separate issues.
Re: iCloud - it's similar to Dropbox. Well suited for document-based apps like OmniGraffle or OmniOutliner, but OmniFocus has different needs. It's going to take some re-engineering of the app to do things the iCloud way.
We may add iCloud support, but since there's a
free sync option already available for OmniFocus, it's not our number one priority. We get "please update the Mac app's UI to be more like the iPad's" requests more frequently, for example. :-)
Background sync: my understanding is that third-party applications cannot initiate sync unless they're in front and running. If Reminders is actually syncing in the background (as opposed to using the push notification system or something) it may not be possible for us to do the same thing. Apple's own apps have access to features and code that other developers don't...