skorvek, if you make a change in OmniFocus on one device, it will automatically sync within a minute, in order to make sure that the sync server is quickly updated. It also syncs immediately if you switch away from OmniFocus. (It only has a brief period available after you switch away before iOS will shut it down, but in typical usage that should be sufficient to complete the sync.) Given this frequency, I think highlighting the sync button when there are changes would mostly serve to distract users and to make them compulsively hit the sync button despite the fact that it would sync in a moment anyway.
I think you'll find that the real issue is that your _other_ client don't know that the data on the sync server has changed, so they don't immediately grab those changes. They will sync once an hour (or at launch, if it's been at least an hour since the last sync) in order to see if there are any changes waiting for them.
Therefore, if it's been over a minute since you used the last client, hitting the sync button when you pick up with a different client should be sufficient to keep you in sync.
That and some other sync info is available on the OmniFocus support page:
http://www.omnigroup.com/products/omnifocus/support/
Of course, you may still want the sync button to change color / highlight in some way when local changes have not been synced to the server. If so I would use the "Send Feedback" menu item in OmniFocus for Mac or in the gear popup in OmniFocus on iPhone, or the "Contact Omni" gear popup item in OmniFocus on the iPad in order to send in that feedback, along with a description of why you think it's still compelling given the existing sync frequency, to ensure it gets recorded and considered along with your use case.