I usually pick Airtran out of MKE as they have a later flight then DL. A few months ago when I arrived at MKE I found out the FL flight was in a maintenace hold and expected to cancel. They agent immediately handed me a document and said to head over to Midwest. They accepted it and I was on my way.
Not sure if this was an interline or what but it was the agent who initiated it and didn't wait to see if the FL flight did cancel. BTW, I checked the monitor in ATL and the FL flight did go quite a bit later.
Kudos to the FL agent.