A number of people have encountered this error, with various applications. There is a discussion with some help at Apple's support site here:
http://discussions.apple.com/thread....46859&tstart=0
The thing that seems to work for most people (not everyone) is to disable syncing of the pertinent application via iTunes, perform a sync (which should completely remove the application from the phone/iPod touch), then re-enable syncing of that application in iTunes and sync again. A more detailed description is here:
http://www.damienkomala.com/v6/unkow...on-the-iphone/
The downside is that this will delete all of your OmniFocus data on your phone, so it's best if you can first get OmniFocus to sync to MobileMe or other WebDAV server so that you can then sync your data back after you get OmniFocus reinstalled and upgraded on your phone.