Even before any recent changes, just about the entire fare (less the fees i guess) is "Lifted" once the first leg takes off.

I know this from experience as in 2004, I was stranded in MDW when they didn't hold the last flight for me and I took ATA home and only got $7.50 back.

Then in 2010 my flight TPA-ISP was diverted to BWI for the night to sleep in the gate area and announcements were made that if anyone tried to get home via other means, no money would be refunded since the "fares were lifted at Tampa".